DN547 - VID Controller with Phase Extender Provides 120A Power Supply for Latest Generation of FPGAs, ASICs and Processors

VID Controller with Phase Extender Provides 120A Power
Supply for Latest Generation of FPGAs, ASICs and Processors
Design Note 547
Mike Shriver
Introduction
The current drawn by FPGAs, ASICs and processors
for high performance servers, network and computing
systems continues to rise, and load currents of 100A
or greater are becoming common. Meanwhile, the
chip's operating voltages are dropping to 0.9V and
below with tighter voltage regulation requirements.
For many of these applications, the core voltage may
need to be adjusted for optimum performance with
a VID (voltage identification) interface. A significant
challenge is clearly placed on the power supply
designer to meet the demands of high efficiency and
tight output voltage regulation with a small amount
of board space.
One way to meet these demands is to use the LTC3877
and LTC3874 chipset. The LTC3877 is a peak current
mode, VID-controlled dual output synchronous
VIN
6V TO
14V
+
VOUT
0.6V TO
1.23V
120A
180µF
×2 L1 PHASE 1
0.25µH
M1
3.57k
0.22µF
CoBlk +
330µF
×12
TG1
INTVCC
LTC3877
SW1
M2
715Ω
VIN
0.22µF
FROM µP
10k
20k
BG1
SNSD1+
SNS1–
TG2
SNSD2+
SNSA1+
SNS2–
VID
VOSNS1+
VOSNS1– SNSA2+
ITH1
DIFFOUT
ITH2
VFB1
+
VFB2
FREQ
–
VFB2
The LTC3874 is a peak current mode phase extender
chip. It does not have an error amplifier and instead
regulates its phase current to the ITH signal from
the LTC3877 master. The elegant design of the
LTC3874 reduces trace count and board space.
The LTC3877 comes in a 44-lead, 7mm × 7mm QFN
package; the LTC3874 slave controller comes in a
28-lead, 4mm × 5mm QFN package.
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and
PolyPhase are registered trademarks of Linear Technology Corporation. All
other trademarks are the property of their respective owners.
10µF
x8
4.7µF
PHASE 2
M3
L2
0.25µH
SW2
BG2
step-down controller. The output of phase 1 can be
programmed from 0.6V to 1.23V in 10mV increments
with a six-bit parallel VID interface. Phase 2 provides
an output of 0.6V to 5V, which is set by an external
divider. The two phases can be paralleled together
or with phases from another LTC3877 or an LTC3874
for higher output current.
VIN
PHASE 3
INTVCC
M5 TG0
L3
0.25µH
LTC3874
TG1
M4
M6
SW0
SW1
BG0
BG1
PHASE 4
4.7µF
M7
L4
0.25µH
768Ω
M8
3.57k
768Ω
0.22µF
8.87k
0.22µF
715Ω
ISNS0+
ISNS1+
ISNS0–
ISNS1–
0.22µF
0.22µF
8.87k
ITH0
100pF
86.6k
3.3nF
47pF
4.02k
ITH1
FREQ
SYNC GND
75k
MODE/
PLLIN CLKOUT GND
NOTE:
THIS IS A SIMPLIFIED SCHEMATIC. REFER TO
THE DATA SHEET FOR A COMPLETE SCHEMATIC.
L1–L4: WURTH 744301025 (0.25μH, 0.32mΩ)
M1, M3, M5, M7: INFINEON BSC050NE2LS
M2, M4, M6, M8: INFINEON BSC010NE2LSI
Figure 1. 4-Phase 120A VID-Controlled Converter Operating at a Switching Frequency of 400kHz
02/16/547
DN547 F01
High efficiency is a result of the strong gate drivers
and short dead times of the two chips, MOSFET
selection and sub-mΩ DCR ferrite inductors. The
full load efficiency for a 1.2V output at 120A load is
88.8% as shown in Figure 2.
95
VIN = 12V
EFFICIENCY (%)
90
85
VOUT = 1.2V
VOUT = 1.0V
VOUT = 0.9V
VOUT = 0.6V
75
70
0
20
40
60
80
LOAD CURRENT (A)
L = WURTH 744301025
0.25μH, 0.32mΩ DCR
VOUT = 0.9V
VIN = 12V
12.0
10.0
8.0
6.0
4.0
2.0
PHASE 1
PHASE 2
PHASE 3
PHASE 4
0
–2.0
0
20
40
60
80
LOAD CURRENT (A)
100
120
DN547 F03
Figure 3. Current Sharing of the 4-Phase 120A
VID-Controlled Converter
to set the internal frequency if synchronization is
not required. The LTC3877 offers three light load
operating modes: Burst Mode ® operation, forced
continuous mode and pulse-skipping mode. The
LTC3874 operates in either forced continuous mode
or pulse-skipping mode.
The minimum on-time of the LTC3877 is 40nsec
typical, ideal for high step-down converters or small
footprint or high bandwidth converters operating
at switching frequencies of 500kHz to 1MHz. The
minimum on-time of the LTC3874 is 90nsec, typical.
L = WURTH 744301025
0.25μH, 0.32mΩ DCR
QT = BSC050NE2LS
QB = BSC010NE2LSI
80
14.0
CURRENT SENSE VOLTAGE (mV)
PolyPhase Design with High Accuracy
and Efficiency
The 4-phase step-down converter shown in Figure 1
uses the LTC3877 and LTC3874 to provide a VIDcontrolled output of 0.6V to 1.23V at a maximum load
current of 120A at a switching frequency of 400kHz.
The LTC3877 yields a total DC regulation accuracy
of ±1% for all VID set points over temperature. The
differential remote sense amplifier in the LTC3877
senses the output voltage at the regulation point and
compensates for voltage drops across PCB trace runs
and ground planes. The 4-phase operation results
in lower output voltage ripple and faster load step
response due to shorter clock delays.
100
120
DN547 F02
Figure 2. Efficiency of the 4-Phase 120A VID Converter
Sub-mΩ DCR Sensing
Both the LTC3877 and LTC3874 use a proprietary
DCR current sensing architecture designed for submΩ DCR sensing, which ensures tight control of the
current sharing and current limit. Figure 3 shows the
current sharing performance of the 4-phase converter
of Figure 1. The inductor used is the Wurth 744301025
(250nH), which has a DCR of 0.32mΩ. The current
sharing error is less than 1mV between phases.
More Features
The LTC3877 and LTC3874 both have a phase-lockable
frequency range of 250kHz to 1MHz and a FREQ pin
Data Sheet Download
www.linear.com/LTC3877
Linear Technology Corporation
Phase 1 of the LTC3877 provides VID control. If
the FPGA, ASIC or processor is not awake or VID
programming is not required, then the VID section can
be disabled by pulling the VIDEN pin low and setting
the output voltage with a divider at the output of the
differential amplifier. Both phases of the LTC3877
have differential remote sense amplifiers for precise
control of the output voltage. The input voltage range
of both chips is 4.5V to 38V.
Other features of the LTC3877 include PGOOD pins,
RUN pins and TK/SS pins for each rail. The LTC3874
has its own RUN pins and FAULT pins for quick
response to fault conditions.
Conclusion
The LTC3877/LTC3874 chipset provides the power
system designer with a highly accurate, efficient
and robust PolyPhase ® solution for high current rails
supplying FPGAs, ASICs and processors.
For applications help,
call (408) 432-1900, Ext. 3270
DN547f LT/AP 0216 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2016
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