DN326 - 3-Phase LTC3733 Provides High Performance Power Solutions for AMD Opteron and Athlon 64 Processors

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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
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