advertisement High Performance Power Solutions for AMD Opteron and Athlon 64 Processors – Design Note 326 Henry J. Zhang and Wei Chen Introduction AMD Opteron and Athlon 64 CPUs can draw 65A at very low voltages (0.8V to 1.55V). Such high currents put efficiency and related thermal issues at the top of a power supply designer’s list of problems. The new LTC ®3733 PolyPhase® synchronous buck controller solves these problems, and others, by enabling solutions that offer high efficiency, low profile and fast transient response. inductor size. The LTC3733’s integrated MOSFET drivers also simplify the design and minimize the overall supply footprint. The LTC3733 directly senses the inductor current of each phase to achieve excellent current sharing among phases, thus evenly distributing the thermal stress. As a result, overdesign of the MOSFETs and inductors is unnecessary and the circuit can be operated reliably without a heat sink. The total size and cost of the solution is minimized. The LTC3733 is a 3-phase current mode controller that drives three synchronous buck stages 120° out of phase. The three out-of-phase stages inherently self-cancel the current ripple and thus minimize the size of the input and output capacitors. Likewise, ripple cancellation and a high switching frequency (up to 600kHz) make it possible to use lower value output inductors, resulting in faster transient-load response and smaller physical The LTC3733 comes in a 36-Lead SSOP package and the LTC3733-1 comes in a low profile 7mm × 5mm QFN package with an exposed ground pad to minimize thermal impedance. An internal differential amplifier provides L, LT, LTC, LTM, Linear Technology, the Linear logo PolyPhase, Burst Mode are registered trademarks and Stage Shedding is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. POWER GOOD VID0 5V VID1 R2 RUN 100k 5% 5V 1 2 3 4 + 5 VOS– 6 VOS C2 1200pF 7 8 9 1S1+ 1S1– 1S2+ 1S2– 1S3– 1S3+ R10 10Ω R11 10Ω 10 C4 1000pF X7R R12 10Ω R13 10Ω 12 13 C5 1000pF X7R 14 15 R14 10Ω R15 10Ω 11 C6 1000pF X7R 16 17 18 VID1 RUN PLLFLTR C9 100pF PGOOD BOOST1 FCB LTC3733CG TG1 IN+ SW1 IN– BOOST2 DIFFOUT TG2 EAIN SW2 SGND VCC SENSE1+ BG1 SENSE1– PGND SENSE2+ BG2 SENSE2– BG3 SENSE3– SENSE3+ SW3 TG3 SS BOOST3 ITH VID4 VID2 VID3 C7 C8 0.01μF 220pF R16 1k X7R R18 22.1k VID0 D1 BAT54A 36 Q1 HAT2168H 35 C14 0.1μF X7R 34 33 Q3 HAT2165H 32 C15 10μF 35V VIN+ L1 0.68μH Q4 HAT2165H D4 B320A C13 0.1μF X7R 31 30 R20 1Ω C11 1μF X5R 27 26 25 5V C12 10μF 10V Q5 HAT2168H Q7 HAT2165H 24 1S1+ 21 C14 0.1μF X7R 20 Q9 HAT2168H D3 BAT54 19 5V C35 22μF 25V C17 10μF 35V VIN+ L2 0.68μH 7V TO 21V Q8 HAT2165H D5 B320A C19 10μF 35V + 1S2+ COUT 1S2– R29 51Ω VIN+ L3 0.68μH R27 0.002Ω DN326 F01 Q11 HAT2165H Q12 HAT2165H VOS+ R28 51Ω R25 0.002Ω D6 B320A 1S3+ R17 71.5k 5V VID2 VID3 VID4 Figure 1. 65A AMD Processor Power Supply Using the LTC3733 11/03/326_conv 1S1– 23 22 J1 VIN+ + J2 VIN– 29 28 R23 0.002Ω 1S3– J3 VOUT+ C48 1μF 0.8V TO 1.55V X5R AT 65A MAX J4 VOUT– VOS– true remote sensing of the regulated supply’s positive and negative output terminals at the load, providing accurate output regulation in high current applications. The internal 5-bit VID programmable attenuator complies with the AMD Opteron and Athlon 64 processors’ VID table (0.8V to 1.55V). The LTC3733 also incorporates two selectable light load operation modes: Burst Mode® operation and Stage Shedding™ mode to significantly improve light load efficiency. With internal overvoltage and overcurrent protection features, the resulting LTC3733 solution provides superior performance for a wide range of applications, from enterprise CPU to mobile CPU applications. 3-Phase, 65A AMD VRM Design Figure 1 shows a 3-phase 65A AMD processor power supply. This supply achieves better than 88% efficiency for a 7V input and a 1.55V/65A output, as shown in Figure 2. Even for a 21V input, efficiency peaks at 96 more than 90%, remaining high throughout a wide 5A to 65A load range. Figure 3 shows a measured load transient waveform. The load current step is 46A with a slew rate of 50A/μs. Ten low profile Sanyo POSCAP capacitors (330μF/2.5V) are used on the VRM output. The maximum output voltage variation during the load transients is just 88mVP-P. Conclusion The LTC3733-based low voltage, high current power supply meets the AMD processor VRM requirement. With 3-phase switching and superior light load operation modes (Stage Shedding and Burst Mode operation), this design can achieve high efficiency, small size and low overall solution cost. Accurate current sharing among the three phases improves thermal performance, and therefore, the long-term reliability of the CPU core power supply. By combining the LTC3733 with one or more LTC3731s, a six or higher phase power supply can be easily obtained to power multiple CPUs. For more information, please contact Linear Technology Corporation. VOUT = 1.55V 94 VIN = 7V EFFICIENCY (%) 92 90 VIN = 21V ΔV 88mV 88 VOUT 100mV/DIV 86 84 IOUT 20A/DIV 82 0 10 40 30 50 20 LOAD CURRENT (A) di/dt 50A/μs ΔI 46A di/dt 50A/μs 60 DN326 F02 Figure 2. Efficiency vs Load Current at 7V to 21V VIN, 1.55V VOUT Data Sheet Download www.linear.com Linear Technology Corporation DN326 F04 Figure 3. Load Transient Waveforms for a 46A Step with a 50A/μs Slew Rate For applications help, call (408) 432-1900 dn326f_conv LT/TP 1103 316.5K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2003