A Novel Predictive Inductor Multiplier for Integrated Circuit DC-DC Converters in Portable Applications

Transcription

1 A Novel Predicive Inducor Muliplier for Inegraed Circui DC-DC Converers in Porable Applicaions Lucas Andrew Milner, Suden Member, IEEE Georgia Tech Analog and Power IC Design Lab 777 Alanic Drive Alana, GA ABSTACT While he large passive elemens of power converers are in he way of converging walls of shrinking cell phones and cameras, he new capabiliies hese devices flaun are creaing addiional burdens and making i difficul o mee specificaions wihou even bigger elemens. Acive circuis ha enhance he effecs of passive elemens will allow power converers o handle larger loads and ge smaller a he same ime. This paper presens a predicive inducor muliplier circui ha amplifies he effecive inducance in a Buck converer. The oupu ripple of he simulaed converer is so small ha he converer appears o have an inducance hiry-eigh imes he value acually used. Compensaing for small inducors inroduces new power losses, bu i is discovered ha linear regulaors and faser swiching converers can be even less efficien. Caegories and Subjec Descripors: B.7.1 [Inegraed Circuis]: Types and Design Syles General Terms: Performance, Design Keywords: Power Managemen, Inegraed Inducors, Inducor Mulipliers, Acive ipple Filers 1. INTODUCTION Porable elecronics are becoming more compac and more versaile o saisfy consumer demand for convenience and syle and o realize indusrial dreams of a world filled wih sensors. While cell phones have swallowed digial cameras, game consoles, and media players wihou gaining any weigh, similarly hin, ligh elecronic organizers now boas of heir alens for wireless web browsing and global posiioning. Furher inegraion could feaure all of his funcionaliy in a single chip and new inelligen sensors embedded in everyhing from clohing o coffee mugs [1], [2]. However, hese dreams imply conflicing requiremens for power managemen circuis, which need large passive elemens o reliably suppor all of hese new feaures. Swiching regulaors use inducors and capaciors o ransform and filer Permission o make digial or hard copies of all or par of his work for personal or classroom use is graned wihou fee provided ha copies are no made or disribued for profi or commercial advanage and ha copies bear his noice and he full ciaion on he firs page. To copy oherwise, or republish, o pos on servers or o redisribue o liss, requires prior specific permission and/or a fee. ISLPED 05, Augus 8 10, 2005, San Diego, California, USA. Copyrigh 2005 ACM /05/ $5.00. Gabriel A. incón-mora, Senior Member, IEEE Georgia Tech Analog and Power IC Design Lab 777 Alanic Drive Alana, GA rincon-mora@ece.gaech.edu supply volages, and hese elemens are already made as small as possible o save space. The sizes of hese componens are inversely proporional o he unwaned ripple on he regulaed oupu, and herefore, as shrinking porable elecronics draw more power o perform more numerous and demanding asks, regulaors will acually need larger passive elemens o keep pace. The use of smaller componens can ranslae ino an unseady and useless supply for subcircuis in he sysem. Linear regulaors avoid his conflic because hey do no require large inducors and capaciors. In a linear regulaor, a ransisor is conneced beween he unregulaed inpu and he regulaed oupu supply. The circui is effecively a variable volage divider, and i yields a seady oupu volage regardless of he curren demanded. However, he oupu curren is always he same as he inpu curren, and his means ha he efficiency of a linear regulaor can never be higher han he raio of oupu o inpu volage. This is a major limiaion in very common siuaions like when a Lihium ion baery in he hree o four vol range provides power for one o wo vol digial circuis. Swiching regulaors are very popular, because hey can efficienly conver supply volages regardless of he conversion raio. One ype is a swiched capacior converer, also called a charge pump. In hese opologies, one or more capaciors are conneced o he source o charge for a period of ime. They are hen conneced o he load where hey deliver he oupu curren and susain he oupu volage. Depending on wheher he capaciors are conneced o he source and load in series or in parallel, hese opologies can produce oupu volages ha are higher or lower han he inpu. These circuis are popular wih designers because hey do no use inducors, which are expensive. However, he conversion raio is fixed by he opology, which makes hem less versaile han he oher ype of swiching regulaors [3]-[6]. conrol (b) (a) Figure 1. A Buck converer (a) is shown wih pulse volage (b) and inducor curren waveforms (c). C ou in L (c) ou L

2 Swiched converers ha use inducors are he mos versaile of power managemen circuis, because he possible conversion raios are unlimied. Figure 1 shows he schemaic of a Buck converer, which provides a low oupu volage from a higher inpu. In his circui, he wo swiches are opened and closed in a complimenary fashion, so ha he volage a he inermediae node,, is a square pulse wih a specific duy cycle. The desired oupu of he circui is he average of his pulse, equal o he produc of he inpu volage and he duy cycle. The inducor and capacior form a second-order filer o reduce he higher frequency componens and leave he oupu volage wih lile or no ripple. Wih few excepions [7], hese passive elemens reside off he chip because of heir physical dimensions. The effecs of smaller inducors and capaciors can be compensaed for by swiching faser, which moves he frequency harmonics in he waveform higher in he specrum where he aenuaing acion of he inducor and capacior combinaion is greaer. This sraegy consumes power in he form of greaer swiching losses and i herefore lowers efficiency. An alernaive is o develop acive circuis ha muliply he effec of he passive elemens. Inducor mulipliers, in paricular, can complimen recen work on inegraed inducor fabricaion echnologies [8]-[10]. The combinaion of inducor mulipliers wih hese larger inducors can allow smaller, sysem-on-chip converers wih equal or beer precision han discree versions. However, he value of such converers o produc designers will also be deermined by he losses incurred by he muliplier. A Buck converer wih an inducor muliplier mus be more efficien han a linear regulaor o be useful, and similarly, he cos of adding he muliplier mus be less han ha of swiching faser. Secion II will discuss differen approaches o inducor mulipliers, Secion III will presen a new circui, and Secion I will evaluae is performance. 2. INDUCTO MULTIPLIES Acive devices could subsiue for he inducor alogeher. In paricular, ransconducance-capacior (-C) filers can reproduce he frequency response of an inducor [11] [12], and hey previously have drawn aenion in his applicaion [13]. However, his echnique requires ha an acive elemen pass he dc curren as one does in a linear regulaor, and a swiching converer ha used i would be even less efficien han one. Consequenly, i makes more sense o enhance he inducor han o replace i. The concep of inducor muliplicaion originaes in he fundamenal relaionship beween curren and volage for an inducor, di v = L. (1) d Dividing he volage across he inducor by a facor has he same effec on he curren ripple as muliplying he inducance by ha facor. This has been called volage mode inducor muliplicaion [13]. In conras, he curren mode approach keeps he volage consan. Then if par of he ripple is redireced from he oupu of he inducor, he effec on oher elemens is he same as using a bigger inducor. The volage mode approach is problemaic for power converers because i requires he addiion of lossy elemens in series wih he inducor [14] or some sor of dc-dc conversion for iself, bu he curren mode approach is promising especially in he conex of a Buck converer, because in ha opology, redirecing lefover ripple amouns o acive ripple filering. Ohers have viewed he problem in his way and had success [15]-[17]. The echniques hey have proposed can be divided several ways, bu he mos imporan disincion is beween hose ha sense he ripple and hose ha predic i, as shown in Figure 2 and Table 1. The former can sense he curren ripple hrough he inducor or he volage ripple a he oupu node, hough in [15] and [16], sraegies for sensing he inducor curren direcly proved o be eiher very difficul or unreliable. In conras, applicaions of he volage sensing sraegy (Figure 2a) have effecively muliplied he inducance by 130 when he oupu ripple is already small [15] and by 10 under harder condiions [13]. conrol conrol 0 I feedback C (a) ck (b) or I feedback (c) C ou C ou Figure 2. The feedback (a) and predicive (b) approaches boh generae complimenary ripple currens (c). The predicive approach (Figure 2b) explois he opology of he Buck converer, and he familiar and simple volage and curren waveforms produced wihin i [17]. I relies on he fac ha while eiher swich in he Buck converer is closed, he volage impressed across he inducor is consan assuming a negligible ripple a he oupu. If he inducance is known, hen he curren hrough i rises or falls a a predicable rae, and a complimenary curren ripple (Figure 2c) can be generaed and injeced a he oupu so ha he volage here is seady. In [17], he inducor gain varies from six o en for a range of load condiions. Table 1 compares he soluions in heir imporan respecs. Since hese are all curren-mode approaches, hey will all consume he same power o cancel he inducor ripple curren. Therefore, he efficiency of a converer should be abou he same regardless of which muliplier is used. The predicive approach is more sable, since i uses a feedforward pah, bu i is probably less accurae han he volage sensing approach for he same reason. Since he difference in sabiliy is inheren, bu he difference in accuracy may no be, he predicive approach is he bes wih which o sar. ck

3 Table 1. Comparison of Acive Mulipliers Curren [15][16] Sensing (Feedback) olage [13][15] Predicing (Feedforward) [17] Area Poor Excellen Excellen riangle Sabiliy Good Good Excellen Efficiency Good Good Good Accuracy Poor Excellen Good in ref bias C ri bias 3. POPOSED CICUIT The curren hrough he inducor in a Buck converer is a riangle wave. I ramps up a a rae proporional o he difference beween he inpu and oupu volages, and i ramps down a a rae proporional o jus he oupu volage. The easies way o generae such a wave is o injec a square pulse ino a capacior, as shown in Figure 3. One level of he pulse should be proporional o he difference beween he inpu and oupu volages, and he oher proporional o jus he oupu volage. K K ( ou ) riangle IC load riangle Figure 3. Concep for building a riangular wave. This is a more precise way of building a complimenary ripple from he informaion available in he converer han he classic inegraor used in [17]. The curren sources can be implemened by forcing he necessary volages across resisors and mirroring he resuling currens as shown in Figure 4. When he curren hrough he inducor is decreasing ( is low), an opamp imposes a reference equal o he oupu volage across a resisor. The resisor curren is mirrored and sourced o he capacior, C riangle. A similar hing happens during he oher half of he cycle, when a consan curren proporional o he difference of and flows ou of C riangle. This process will creae a riangular volage waveform across C riangle as long as ha waveform does no clip a or ground. For his reason, riangle is biased a an inermediae volage, bias. Figure 4. Implemenaion of conrolled curren sources. Any error in he currens ha alernaely flow in and ou of C riangle will cause he dc level of he volage across i, riangle o drif owards or ground. Adding a large resisor in parallel wih C riangle will preven ha bu i will also allow a dc offse o reach he ransconducor. If his happens, he ransconducor will conduc a dc curren as par of he load curren ha he inducor would normally carry alone, and he efficiency of he converer will decrease. Consequenly, i is imporan o limi any offses in he circui leading o he ransconducor. One source of mismach is he pair of opamps ha may have differen inpu offse volages. If one opamp is used o alernaely conrol boh mirrors his source of error is neuralized. Figure 6 shows how he mirrors can share he opamp wih he aide of four swiches. The mirrors hemselves may have dc offses, and so a highpass filer is added o he combinaion of C riangle and a large resisor, as shown in Figure 6. The complee filer mus filer dc offses and inegrae he square curren pulse as a single capacior would and as shown in Figure 5. riangle (db) Aenuae DC offses o save power Swiching Frequency Inegrae o creae riangular volage (capaciaive effec) Frequency (Hz) Figure 5. Filer frequency response.

4 Op-Amp Sharing Filer riangle in ref bias Figure 6. Implemenaion of Op-Amp sharing and addiional filering. 4. SIMULATION ESULTS An open-loop Buck converer wih a 1µH inducor and a 10µF capacior was simulaed a 100kHz wih a fify percen duy cycle and a 1A load curren. Wihou he inducor muliplier, he oupu volage had a very large ripple as shown in Figure 7. Tha ripple is hiry-eigh imes smaller wih he inducor muliplier. () 31 mpp Wihou inducor muliplier pp The aenuaed ripple has new frequency componens, because he synhesized ripple does no vary from an ideal riangle wave in he same way ha he inducor ripple does. This imperfec cancellaion is shown in Figure 8. Ineresingly, he inducor curren ripple is decreased by a facor less han weny, which is slighly more han half he facor by which he oupu volage ripple of he converer has been decreased. The capacior makes up he difference because i filers a higher frequency curren ripple ( + ) han normal ( ). Oher han he muliplicaion facor, he mos imporan meric for he inducor muliplier is he new efficiency of he Buck converer. However, his is dependen on he size of he inducor curren ripple and he efficiency of he original converer. This makes comparing inducor mulipliers o linear regulaors and faser converers a projec by iself. In an earlier sudy involving one simulaed converer, decreasing he inducance and adding he inducor muliplier reduced he efficiency of he original converer from 86% o 74% [13]. A linear regulaor designed for he same conversion raio would have been 68% efficien, while a converer swiching fas enough o produce he same low ripple would have been 78% efficien. However, using he inducor muliplier insead of a higher swiching frequency can acually save power. I can be shown ha when swiching losses are already high, increasing he swiching frequency o reduce he ripple consumes more power han generaing a complimenary ripple. (A) 1A average Δ I = 7.14A Wih inducor muliplier Time (s) (A) 0A average Δ I = 7.00A Figure 7. Oupu volage wih and wihou he muliplier. + (A) Δ I = 368mA 1A average Time (s) Figure 8. Inducor curren, predicive curren, and sum.

5 To begin, swiching losses are proporional o frequency as shown in Eq. 2, P swiching = in I load x f. where x is he combined rise and fall ime of he swiches in he Buck converer. The losses inroduced by he muliplier are deermined by he inducor curren ripple and he supply volage of he ransconducor (), as shown in Figure 9. I Top I Boom I Top I Boom Δ 2 Δ 2 Δ 8 Δ 8 (2) An inegraed converer migh have he following specificaions: =3, =1.5, L=1µH, C=1µF, I ou =2A, HS =75mΩ, LS =10Ω, ESL = ESC =10mΩ, and x =20ns. In his case, if he proposed circui muliplies he effecive inducance by a facor of hiry, hen a similar converer wihou he muliplier would have o swich five and a half imes faser o achieve he same accuracy. As shown in Table 2, he inducor muliplier inroduces 280mW of unique losses (LMX), bu swiching losses (SW) in he oher converer sar a 120mW and muliply by 5.5. The circui wih he inducor muliplier is more han 5% more efficien. A consequence of he above equaions is ha larger on-chip inducors, higher swiching frequencies, and higher loads all increase he advanage of inducor mulipliers in relaive efficiency. The inducance used in he example is larger han can be produced on chip, bu only o demonsrae clearly he advanage of inducor muliplicaion. Even hough inegraion ono he IC is he ulimae purpose, his echnique would also save board area by reducing he necessary inducance o ha size. Also, he feasibiliy and cos of applying he echnique o every inducor on chip is no here considered, because one main power inducor for he enire chip is envisioned, and he inducor curren waveforms in oher applicaions may no be as simple o process. Figure 9. Transconducor () sage and curren waveforms. +k Muliplier Each ransisor in he oupu sage will have o conduc half of he synheic complimenary ripple each period, and he average curren hrough hem will be equal. The volage across he op ransisor is always he difference in he inpu and oupu volages, while he volage across he boom is jus he oupu volage. Summing he consan volages across he ransisors and muliplying by he average curren hrough hem gives he oal power los in hese elemens, P muliplier Table 2. Example Of Converer Losses Faser Swiching = Inducor Muliplier Big Discree Inducor f 5.5MHz 1MHz 1MHz Δ 136mA 750mA 25mA DC 224mW 224mW 224mW MS 1.02µW 3.09mW 3.44µW SW 660mW 120mW 120mW LMX 0mW 280mW 0mW Toal 884W 628mW 344mW Efficiency 77.2% 82.7% 89.7% in ΔI 8 L. (3) k L buck Figure 11. Cancellaion of forward pahs. A disadvanage of using he predicive inducor muliplier is degradaion of he ransien response. As shown in Figure 11, he added pah from o, hrough he inducor muliplier cancels he original pah hrough he real inducor, so ha no race of he ac componen of appears a. This is, afer all, he poin of he inducor and he inducor muliplier. However, if he wo pahs cancel perfecly, he converer canno respond o load ransiens as he conrol loop from o and back o is effecively open. Essenially, as he effecive inducance increases owards infiniy so does he seling ime of he converer. The effec would be no differen if a large inducor were used. I is he same radeoff ha designers already face. However, he possibiliy remains ha he inducor muliplier can be swiched ou of he circui, leaving only a small inducor and a fas converer for large ransiens. This may eliminae he radeoff alogeher. 5. CONCLUSIONS There are clearly cases in which inducor mulipliers are he mos efficien way o inegrae dc-dc converers on chip. This implemenaion of he predicive echnique muliplies he inducor almos fory imes. Defining he se of cases in which is use is appropriae is difficul, however. Generally, when he iniial ripple is already low, an inducor muliplier will furher aenuae i more efficienly han increasing he swiching frequency can. Progress in on-chip and on-package inducor fabricaion echnologies consequenly favors inducor mulipliers. Because of i, he siuaions in which dc-dc converers are preferable o linear regulaors will become more numerous, and inducor mulipliers will help o

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