ECE 497 JS Lecture 16 Power Distribution

Transcription

1 ECE 497 JS Lecture 16 Power Distribution Spring 2004 Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois 1

2 Overview Motivations & Objectives Power Supply Network Inductive Noise Resistive Noise Bypass Capacitors Regulators On-Chip Power Distribution Power Supply Isolation Examples 2

3 Motivation and Objectives Provide stable, quiet DC supply voltage Compensate for large AC current draws Compensate for fast transients Current draws of 200A Rate of change of 200 GA/s Voltage supply needs to be maintained within 10% 3

4 IC on Package 4

5 Power-Supply Noise - Power-supply-level fluctuations - Delta-I noise - Simultaneous switching noise (SSN) - Ground bounce VOH VOL Ideal Vout Actual Vout Time 5

6 Power Distribution Path 6

7 Power Supply Network Bus Backplane PC Board Package IC Interconnect Power Supply Load On board inductance and on-chip resistance Symmetry between power and ground (return path) Distributed over several levels of interconnections 7

8 Load Current Load is usually periodic with clock Can be in phase and reinforce one another Load are often resistive, varying linearly with supply voltage Load can be high impedance Current Time 8

9 Local & Signal Loads Local loads connect a point in the network to a corresponding point in the ground network Current can be supplied by local bypass capacitor Signal loads connect a point in the power network to a distant point in the ground network Unbalanced signaling Long return current path Bypass capacitors do not help 9

10 Inductive Supply Noise Each section of the circuit is an LC tank - Resonant frequency is ω LC =(LC) -1/2 - Inductor carries DC current (low pass) - Capacitor supplies AC current (high-pass) Chose size of capacitor to: - supply cycle to cycle AC current with acceptable ripple - handle inductor start/stop transient 10

11 Local Regulation Used to prevent overshoot so voltage cannot exceed nominal value by more than a small amount. Supply overshoot can be reduced via clamping Supply voltage droops can be reduced using shunt regulators Clamps draw little power and are inexpensive Shunt regulators dissipate considerable averge power and are expensive 11

12 Local Regulation Using Clamps L I 1 I L V p I X (V L, I 1 ) C B + LOAD - V L I X 0 if VL < Vn = k ( V - V ) if V > V s L n L n k s : transconductance of clamp Clips off top half cycle by directing inductor current into clamp rather than capacitor prevents overshoot. Cannot prevent supply voltage drooping. 12

13 Shunt Regulators L I 1 I L V p I X (V L, I 1 ) C B + LOAD - V L [ ] I = max 0, I I + k ( V V ) X max 1 s L n k s : transconductance Keeps current constant Regulates voltage Not used on chip Power hungry and expensive Last resort to prevent supply voltage droops 13

14 General Topology for Power Distribution Hierarchy of distribution networks Usually a tree sometimes a loop Upper level inductive with distributed caps On-chip level resistive with distributed caps 14

15 Role & Function of Bypass Capacitors Inserted between power and ground in path between supply and load Supply AC current to load faster than inductor can respond Can be distributed or lumped intermediate between a transmission line and and an LC circuit In reality includes some resistance and inductance 15

16 Models for Inductive Section of Supply Network Distributed Lumped 16

17 LC Section Transient Response For 1 st 10 ns cycles, the load draws an average current of 0.12A After 5 th cycle, the load current is shut off to accept step k C i B = kq = = V max t ki t V i ck i av ck t 0 ( I I ) dt I avg t avg ck k i : maximum fraction of Charge transferred each cycle k i =1 for delta function k i =0 for DC typical: 0.25 < k i <0.5 17

18 LC Section Transient Response 40 nh & 10nF bypass cap fc=8 GHz 18

19 Natural Frequency L C - LC tank will resonate at natural frequency Iavg V = Cω = I avg C sin( ω t) C L sin( ωct) C V = max I avg L C 19

20 Frequency Range for Bypass Capacitors Capacitors at low frequencies Actually an RLC circuit Resonance frequencies LC frequency RC frequency Ineffective at either of these frequencies 20

21 Natural Frequency of Bypass Capacitors Load currents at frequencies well below ω c see an inductive impedance. Load currents at high frequencies see a capacitor. At ω c, impedance is infinite At ω c, even small currents will cause oscillations 21

22 Typical Bypass Capacitors C R S L C F RC F LC F LR On-chip MOS 0.35 x 114 mm) On-chip MOS (1.4 x 115 µm) 250 ff 10 Ω 0 64 GHz 1pF 40 Ω 0 4 GHz SMT ceramic 1nF 0.1 Ω 1nH 160 MZ SMT ceramic 10 nf 0.1 Ω 1 Ω 1nH 50 MHz Ceramic disk 10 nf 0.1 Ω 5nH 23 MHz Aluminum electrolytic 10 µf 1 Ω 10 nh 160 khz 16 MHz Aluminum electrolytic 1000µF 0.05 Ω 10 nh 3 khz 800 khz 22

23 Bypass Capacitor & series Regulator L V P V L C B I S I L V P L Reg V L C B IL 1A 0A 10 ns 50 ns L= 10 nh What value of C B will keep V L to 5% with - No regulator - Series regulator 3.3V to 2.5V 23

24 Bypass Capacitor & series Regulator 1A 0A 10 ns Q cap I avs 50 ns No regulator V=125 mv I av =200 ma Q cap =6.4 nc C B > 76.8 nf With regulator V=925 mv I av =200 ma Q cap =6.4 nc C B > 7.39 nf P supply =660W 24

25 Bypass Capacitor Network Design (D&P 5-12) - Hold voltage ripple to within 5% of supply voltage - DC supply of 3.3V - Generator internal inductance 1 mh 10 A 2 ns 6 ns 1) V in AC mode < 165V 2) V Ldrop + V Cdrop < 165 mv 3) Capacitor must be operational above breakpoints L source =1 µh Solution L rank3 =166 ph L rank2 =83.3 ph R rank1 = V - R rank3 =0.017Ω R rank2 =0.0083Ω I Load C rank3 =600µF C rank2 =120nF C rank1 =60nF 25

26 I ave 10A 1ns = = 1.67A 6ns Q = (1ns ns)( A) = 6.94nC cap 1 st Rank Average current and charge sourced by capacitor: In AC mode the V of the cap should be less than 165 mv, so: Qcap 6.94nC C > rank1 42nF V = 165mV = Drop in series L must be less than 165mV V 165mV Lrank1 < = = 16.5pH di / dt 10A 1ns 26

27 1 st Rank - Need breakpoints above 1 GHz to insure true capacitor - From table, choose 60,000 1pF MOS on-chip cap (min:42,000) C = 60 nf, L = 0 rank1 rank1 R rank 1 40Ω 60,000 4 = = Ω resistance is negligible 27

28 2 nd Rank V Lrank+ 1 < Crank Iave 2 165mV Lrank 2 < 60nF = 586 ph 1.67A 2 Cannot connect the first rank up to the supply voltage since supply inductance is 1µH and does not satisfy criterion Choose 12 SMT ceramic caps satisfies inductance calculations And doubles 1 st rank cap. C = 120 nf, L = 83.3pH rank 2 rank 2 R rank 2 0.1Ω = = Ω 12 28

29 3 rd Rank 165mV Lrank 3 < 120nF = 1.17nH 1.67A Since this is less than the inductance of the supply, need to add 3 rd rank of caps Use 11 aluminum electrolytic caps 10nH Crank 3 = 110 µ F, Lrank 3 = = 909 ph 11 1Ω R rank 3 = = 0.091Ω Ω 10A = 910mV V = max 165 mv 2 This resistance looks high, need to determine the associated voltage drop NO GOOD 29

30 Need to reduce resistance to: 3 rd Rank R rank 3 165mV = = Ω 10A Choose 60 aluminum electrolytic caps 10nH Crank 3 = 600 µ F, Lrank 3 = = 166 ph 60 R rank 3 1Ω = = Ω 60 30

31 4 th Rank 165mV Lrank 4 < 600µ F = 5.86µ H 1.67A 2 The inductance of the supply voltage satisfies this criterion no need for 4 th rank. L source =1 µh L rank3 =166 ph L rank2 =83.3 ph R rank1 = V - R rank3 =0.017Ω R rank2 =0.0083Ω I Load C rank3 =600µF C rank2 =120nF C rank1 =60nF 31