Distributed Power Delivery for Energy Efficient and Low Power Systems

Transcription

1 Distributed Power Delivery for Energy Efficient and Low Power Systes Selçuk Köse Departent of Electrical Engineering University of South Florida Tapa, Florida Eby G. Friedan Departent of Electrical and Coputer Engineering University of Rochester Rochester, New York Abstract With the introduction of ultra-sall on-chip voltage regulators, novel design ethodologies are needed to deterine the location of these on-chip power supplies and decoupling capacitors. In this paper, the optial location of the power supplies and decoupling capacitors is deterined for different size and nuber of coponents. Facility location probles are applied to deterine the optiu location of power supplies and decoupling capacitors in the proposed ethodology. I. INTRODUCTION Power consuption has becoe one of the priary design bottlenecks with the proliferation of obile devices as well as server fars where the perforance per watt is the priary benchark [1], [2]. The power generated and regulated by the off-chip and on-chip voltage regulators is distributed to billions of load circuits throughout a power distribution syste. Due to the parasitic ipedances of the power distribution networks, voltage fluctuations in the supply voltage occur. These fluctuations depend on the characteristics of the load current deand and the behavior of the power distribution network. The power supplies are also supported by locally distributed decoupling capacitors which serve as a reservoir of charge to provide current to the load circuits [3]. The coplexity of the high perforance power delivery systes has increased significantly with the integration of diverse technologies on a single die, foring an heterogeneous syste. The required supply voltage levels and the noise constraints vary significantly for different technologies. Novel voltage regulator topologies [4] [10] have recently been proposed, enabling not only on-chip power supply integration but also ultiple on-chip point-ofload power supplies [10] [12]. These on-chip point-of-load power supplies provide the required voltage close to the load circuits, greatly reducing the effective ipedance between the load circuits and power supplies [13]. Next generation power delivery networks for heterogeneous circuits will contain tens to hundreds of on-chip power supplies supported by thousands of on-chip decoupling capacitors to satisfy the current deand of billions of load circuits. The design of these coplex systes would be enhanced This research is supported in part by the National Science Foundation under Grant Nos. CCF and CCF , grants fro the New York State Office of Science, Technology and Acadeic Research to the Center for Advanced Technology in Electronic Iaging Systes, and by grants fro Intel Corporation and Qualco Corporation. if available resources such as the physical area, nuber of etal layers, and power budget were not severely liited. The continuous deand over the past decade for greater functionality within a sall for factor has iposed tight resource constraints while achieving aggressive perforance and noise targets [14]. Several techniques have been proposed for efficient power delivery systes, typically focusing on optiizing the power network [14], [15] and the placeent of the decoupling capacitor [16], [17]. Recently, Zeng et al. [18] proposed an optiization technique for designing power networks with ultiple on-chip voltage regulators. The design tradeoffs of on-chip voltage regulators and the effect of these regulators on high frequency voltage fluctuations and id-frequency resonance have been analyzed. The interactions between the power supplies and the decoupling capacitors are, however, not considered, which can significantly affect the perforance of an integrated circuit [18]. These interactions are quite critical in producing a robust power distribution network [10]. Decoupling capacitors and on-chip power supplies exhibit several distinct characteristics such as the response tie, area requireents, and parasitic output ipedance. Circuit odels for these coponents should accurately capture these characteristics while being sufficiently siple to not overly coplicate the optiization process. In this paper, facility location optiization algoriths will be analyzed to deterine the optiu location of power supplies and decoupling capacitors to iniize power noise [19] [21]. The constraints of this power network co-design proble depend on the application and specifications of the perforance objectives. The optiization goal can be to iniize the axiu voltage drop, average voltage drop, total area, response tie for particular circuit blocks, or total power consuption. Multiple optiization goals can also be applied for saller or id-size integrated circuits. The rest of the paper is organized as follows. A recently developed point-of-load voltage regulator is briefly described in Section II. The facility location proble is introduced with soe exeplary applications in Section III. A proposed ethodology to deterine the optiu location of the power supplies and decoupling capacitors is described in Section IV. The optiu location of the power supplies and decoupling capacitors, deterined for a saple circuit, is presented in /12/$ IEEE 757 Asiloar 2012

2 Output test pad PWM Op ap 80 µ Passive RC 185 µ Op ap output stage Fig. 1. Microphotograph of the hybrid voltage regulator [8]. Section V. The paper is concluded in Section VI. II. POINT-OF-LOAD VOLTAGE REGULATORS Placing ultiple point-of-load power supplies is challenging since the area occupied by a single power supply should be sall and the efficiency sufficiently high. Guo et al. proposed an output capacitorless low-dropout regulator which occupies on-chip area [6]. The authors also recently proposed a hybrid point-of-load voltage regulator, occupying on-chip area [8]. A icrophotograph of this hybrid point-of-load regulator is shown in Fig. 1. These area efficient voltage regulators provides a eans for distributing ultiple local power supplies across an integrated circuit, while aintaining high current efficiency and sall area. With the proposed voltage regulator, on-chip signal and power integrity is significantly enhanced while providing the capability for distributing ultiple power supplies. Design ethodologies are therefore required to deterine the location, size, and nuber of these power supplies and decoupling capacitors. III. FACILITY LOCATION PROBLEM Every coplex syste is coposed of sall coponents, typically with siple structures. The interactions and aggregation of these coponents for a highly coplex syste. The efficiency of this syste strongly depends upon the physical location of these coponents, which significantly affects the interactions. In ost systes, these coponents can be grouped into two categories; (1) facilities, and (2) custoers. The location, size, and nuber of facilities that iniize the cost of providing a high quality service to the custoers are the design objectives [19]. Matheatical odels of the location have been used to deterine the optial nuber, location, and size of the facilities as well as allocate facility resources to the custoers that iniize or axiize the objective function [19] [21]. The proble can be categorized depending upon the network (discrete or continuous) and the input (static or dynaic). The objective is to iniize the average (or axiu) distance fro the facilities to the custoers, deterine the iniu nuber of facilities that serve a particular nuber of custoers at fixed locations, or axiize the iniu distance fro a facility to the custoers. The design of on-chip power delivery networks for heterogeneous circuits exhibits significant siilarities to the design of electrical distribution networks in larger scale systes, such as the electric power distribution grid of a city. The electricity generated at a power plant is downconverted and distributed to substation transforers, typically outside a city. The output voltage of these substation transforers is further downconverted and regulated by the local power supplies. This voltage can be either delivered to industrial custoers at a high voltage level or further downconverted and regulated at saller substations and distributed to the local city power grid. Large capacitors are integrated within this electrical distribution syste to reduce voltage fluctuations. Alternatively, in an heterogeneous integrated circuit, the on-board voltage regulators downconvert the output voltage of the board level power supply unit. This voltage is delivered to the onchip voltage regulators or directly to the on-chip power grid which provides current to the load circuits. The required voltage levels and noise constraints are technology and design dependent. The on-chip power delivery syste is designed to deliver different voltage levels within noise constraints. Decoupling capacitors are distributed throughout the on-chip power delivery network to support the power distribution syste. A parallel can be drawn between the transforers and off-chip voltage regulators, the sall substations and onchip voltage regulators, and the large capacitors and on-chip decoupling capacitors. Additionally, the voltage requireents of different technologies within an heterogeneous integrated circuit vary in a siilar anner as the voltage requireents of industrial and residential regions within a city. Several optiization algoriths have been proposed which consider possible constraints to provide an optial solution to this proble. Due to the siilarity between the electrical distribution network of a city and the power distribution network of a heterogeneous circuit, analogous algoriths can be applied to the design of these systes. Since facility location algoriths are widely used to design electrical distribution networks, these city planning algoriths are leveraged in designing on-chip power networks within heterogeneous circuits. IV. PROPOSED OPTIMIZATION METHODOLOGY These existing optiization techniques and ethodologies provide a near optial solution for the location of the onchip power supplies and decoupling capacitors that iniize the average (or axiu) power noise. The focus of this paper is to deterine the optial nuber and location of the on-chip power supplies and decoupling capacitors that iniize the average power noise. A closed-for odel of the ipedance, proposed in [13], is utilized to deterine the effective resistance fro the power supplies and decoupling 758

3 capacitors to the load circuits. The constraints of the proble are as follows, The total area of the power supplies and decoupling capacitors is aintained constant All power supplies ust be larger than the iniu sized power supply All decoupling capacitors ust be larger than the iniu sized decoupling capacitor The proposed objective function is Miniize F(n,,k) = K 1 +K 2 +K 3 Subject to n C Pij (R out (P i )+R eff (P i,l j )) k C Dij (R esr (D i )+R eff (D i,l j )) k cap Di N trlj (R esr (D i )+R eff (D i,l j )), R eff (node α,node β )/r = 1 2π [ln((x 1 x 2 ) 2 +(y 1 y 2 ) 2 ) ] , (1) (2) 1 < x α,β < (Grid size) X, (3) 1 < y α,β < (Grid size) Y, (4) C Pij = C Dij = G ij n G, (5) ij G ij k G, (6) ij C Pij cap Pi, (7) j=1 C Dij cap Di, (8) j=1 n C Pij + n cap Pi + k C Dij = I i, (9) k cap Di I i, (10) where the definition of the aforeentioned paraeters are listed in Table I. In heterogeneous systes, where the slew rate, operating frequency, and supply voltage vary significantly over different portions of a circuit, K i provides the flexibility to optiize the power distribution syste for different technologies. Paraeter TABLE I DEFINITION OF THE PARAMETERS IN (1)-(10). Definition P i i th power supply D i i th decoupling capacitor L i i th circuit block R eff (node 1,node 2 ) Effective resistance between node 1 and node 2 (x 1,y 1 ) Coordinates of node 1 (x 2,y 2 ) Coordinates of node 2 r Unit resistance within the power grid n Nuber of power supplies k Nuber of decoupling capacitors Nuber of load circuits R out(p i ) Output resistance of i th power supply R esr(d i ) Effective series resistance of i th decap G ij Equivalent conductance K i Weighting paraeter C Pij Contribution of i th power supply to j th load C Dij Contribution of i th decap to j th load cap Pi Capacity of i th power supply cap Di Capacity of i th decap N trlj Noralized transition tie of the j th load circuit I i Current deand of i th load (Grid size) X Power grid size in horizontal direction (Grid size) Y Power grid size in vertical direction V. CASE STUDY The voltage drop aps of the related circuits with the decoupling capacitors and power supplies located at the predeterined locations are obtained using SPICE. The node voltages, which are deterined by the SPICE siulations, are produced fro MATLAB. The optiu nuber and location of the power supplies and decoupling capacitors that iniize the voltage drop and response tie within certain blocks are deterined for a sall saple circuit, as shown in Fig. 2, to provide an intuitive understanding of the proposed ethodology. The saple circuit is coposed of nine circuit blocks with different current profiles. The third and seventh blocks have current profiles with a faster transition tie (i.e., ) than the rest of the circuits which have a relatively slower transition tie (i.e., ). Since the decoupling capacitors provide iediate charge, intuitively, the decoupling capacitors should be placed close to those blocks with a fast transition tie to provide a fast response to transient changes in the current. The optiu location of the power supplies and decoupling capacitors that iniizes both the axiu voltage drop and response tie for certain blocks (the third and seventh blocks) is used, where K 1, K 2, and K 3 are set to one. The optiu location of one large on-chip power supply and ten decoupling capacitors (case a) is shown in Fig. 2a. The power supply is located at a central location to reduce the axiu physical distance to each of the circuit blocks. The decoupling capacitors, however, are placed physically close to the third and seventh blocks. Most of the current deand of these blocks is provided by the surrounding decoupling capacitors. The optiu location of the four relatively low current power supplies and 20 sall 759

4 decoupling capacitors (case b) is also deterined, as shown in Fig. 2b. In this case, the third and seventh circuit blocks are surrounded by local decoupling capacitors whereas the power supplies are distributed to ensure that the axiu distance fro the power supplies to the reaining blocks is iniized. The voltage drop ap for these two cases is shown in Fig. 3, where increasing the nuber of power supplies and decoupling capacitors significantly reduces the voltage drop. The axiu voltage drop is 133 V and 77 V, respectively, for cases a and b. More than a 40% reduction in the axiu voltage drop is achieved by increasing the nuber and distributing the location of the power supplies and decoupling capacitors. The area of an on-chip power supply is typically doinated by the output pass transistors [22], where the size of these pass transistors changes linearly with the axiu output current deand. The size of an on-chip power supply therefore changes linearly with the axiu output current capacity. Additionally, when the on-chip power supplies are sufficiently sall, the ultra-sall power supplies are cobined to for a larger power supply with a higher output current. In this paper, the size of a power supply is assued to change linearly with the axiu output current capacity. The general algebraic odeling syste (GAMS) is used as the optiization tool [23]. The proposed optiization ethodology is odeled as a ixed integer nonlinear prograing proble. The location of the power supplies and decoupling capacitors that iniizes the objective function is deterined for different nuber of power supplies and decoupling capacitors. The total area of the power supplies and decoupling capacitors is aintained the sae for all of the test cases to provide a fair coparison. 20 A #3 #2 20 A #1 20 A 20 A #2 20 A #1 20 A #3 20 A #5 #6 #9 20 A #8 20 A 20 A #4 #7 20 A (a) 20 A #6 #9 20 A 20 A Large power supply #5 #8 20 A 20 A Large decap Sall power supply Sall decap #4 #7 20 A 20 A VI. CONCLUSIONS The siilarity between the facility location proble and the design of heterogeneous integrated circuits is exploited. An objective function based on the effective resistance between the power supplies, decoupling capacitors, and load circuits is proposed that iniizes the average voltage drop throughout a heterogeneous integrated circuit. This objective function considers the contribution of current fro different power supplies and decoupling capacitors to a circuit block as well as the size of the individual circuit blocks. The optial location of the on-chip power supplies and decoupling capacitors is deterined for a saple circuit. REFERENCES [1] D. Meisner et al., Power Manageent of Online Data-Intensive Services, Proceedings of the ACM International Syposiu on Coputer Architecture, pp , June [2] R. Jakushokas, M. Popovich, A. V. Mezhiba, S. Kose, and E. G. Friedan, Power Distribution Networks with On-Chip Decoupling Capacitors, Second Edition, Springer, [3] M. Popovich, M. Sotan, A. Kolodny, and E. G. Friedan, Effective Radii of On-Chip Decoupling Capacitors, IEEE Transactions on Very Large Scale Integration (VLSI) Circuits, Vol. 16, No. 7, pp , July (b) Fig. 2. Floorplan of the exaple circuit with two different power delivery networks, a) one large power supply with ten decoupling capacitors, and b) four relatively saller distributed power supplies with 20 sall decoupling capacitors. [4] K. N. Leung and P. K. T. Mok, A Capacitor-Free CMOS Low- Dropout Regulator with Daping-Factor-Control Frequency Copensation, IEEE Journal of Solid-State Circuits, Vol. 38, No. 10, pp , October [5] P. Hazucha et al., Area-Efficient Linear Regulator with Ultra-Fast Load Regulation, IEEE Journal of Solid-State Circuits, Vol. 40, No. 4, pp , April [6] J. Guo and K. N. Leung, A 6-µW Chip-Area-Efficient Output- Capacitorless LDO in 90-n CMOS Technology, IEEE Journal of Solid-State Circuits, Vol. 45, No. 9, pp , Septeber [7] Y. Raadass, A. Fayed, B. Haroun, and A. Chandrakasan, A Copletely On-Chip Switched-Capacitor DC-DC Converter Using Digital Capacitance Modulation for LDO Replaceent in 45n CMOS, Proceedings of the IEEE International Solid-State Circuits Conference, pp , February [8] S. Kose and E. G Friedan, An Area Efficient Fully Monolithic Hybrid Voltage Regulator, Proceedings of the IEEE International Syposiu on Circuits and Systes, pp , May/June

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