4-Phase Power Supply Delivers 120A in Tiny Footprint, Features Ultralow DCR Sensing for High Efficiency

4-Phase Power Supply Delivers 120A in Tiny Footprint,
Features Ultralow DCR Sensing for High Efficiency
Yingyi Yan, Haoran Wu and Jian Li
The LTC3875 is a feature-rich dual-output synchronous
buck controller that meets the power density demands
of modern high speed, high capacity data processing
systems, telecom systems, industrial equipment and DC
power distribution systems. The LTC3875 delivers high
efficiency with reliable current mode control, ultralow DCR
sensing and strong integrated drivers in a 6mm × 6mm
40-pin QFN. Multiple LTC3875s can be paralleled to provide
higher current, or it can be combined with the LTC3874
to deliver the same performance with a smaller footprint.
The LTC3874 is a small footprint
(4mm × 5mm QFN), dual PolyPhase® current mode synchronous step-down slave
controller (phase extender). It is suitable
for high current, multiphase applications
when paired with a companion master
controller, such as LTC3875. The LTC3874
can use sub-milliohm DC resistance
power inductors to optimize efficiency.
Immediate response to system faults
guarantees reliability of the total solution.
1V V OUT, 120A CONVERTER WITH
PARALLEL LTC3875s
The LTC3875 can be easily configured as
dual-phase, single-output operation for
high current outputs. This design can
be expanded with more converters and
phases in parallel for even higher current.
Figure 1 shows a 4.5V~14V input, singleoutput application schematic using two
LTC3875s. The LTC3875s’ four channels run
with 90° phase shift, reducing input RMS
current ripple and required capacitor size.
Figure 1. A single-output,
4-phase (1.0V/120A) converter
Each phase supports 30A of current with
one top MOSFET and one bottom MOSFET.
The LTC3875 employs a unique current
sensing architecture to enhance its signalto-noise ratio, enabling current mode
control even with a small sense signal
from a very low inductor DCR—1mΩ
or less. As a result, efficiency is high and
jitter is low. Current mode control yields
fast cycle-by-cycle current limit, current
sharing and easy feedback compensation.
The LTC3875 can sense a DCR value as
low as 0.2mΩ with careful PCB layout.
The LTC3875 uses two positive sense
pins SNSD+ and SNSA+ to acquire signals. The filter time constant of the
SNSD+ should match the L/DCR of the
output inductor, while the filter at SNSA+
should have a bandwidth five times
larger than that of SNSD+. Moreover, an
35
EFFICIENCY (%)
90
85
80
75
VIN = 12V
VOUT = 1V
fSW = 400kHz
70
65
0
20
40
60
80
100
VIN = 12V
VOUT = 1.0V
IOUT = 120A
200 LFM airflow
10 | July 2015 : LT Journal of Analog Innovation
Figure 3. Thermal scan of 4-channel regulator
VIN = 12V
VOUT = 1V
30
25
20
15
10
CHANNEL 1
CHANNEL 2
CHANNEL 3
CHANNEL 4
5
0
120
ILOAD (A)
Figure 2. Efficiency of circuit in Figure 1
INDIVIDUAL CHANNEL CURRENT (A)
95
0
20
40
80
60
ILOAD (A)
100
120
Figure 4. DC current sharing is balanced among the
four channels, even at very high current loads
design features
2.2Ω
10µF
×2
4.7µF
D1
VIN
TG2
MT2
0.25µH
DCR = 0.32mΩ
VOUT
INTVCC
BOOST2
0.1µF
L2
PHASMD
SNSD2+
220nF
715Ω
220nF
SNS1–
SNSA2+
SNSA1+
VOSNS2+
ITH1
VOSNS1–
ITH2
VOSNS2–
IFAST
TK/SS1
EXTVCC
TK/SS2
PGOOD
FREQ
220pF
100k
TAVG
13.3k
220nF
220nF 715Ω
100k
3.01k
ILIM
TRSET2
4.02k
330µF
2.5V
×12
VOSNS1+
RUN2
CLKOUT
0.1µF
+
100µF
6.3V
×14
3.57k
SNSD1+
RUN1
2.2nF
ENTEMPB
SNS2–
L1
MB1
BG1
LTC3875
TCOMP1
TCOMP2
VOUT
1V
120A
0.25µH
DCR = 0.32mΩ
0.1µF
SW1
BG2
3.57k
1µF
MT1
TG1
BOOST1
SW2
MB2
10µF
×2
D2
VIN
4.5V TO 14V
20k
TRSET1
1k
MODE/PLLIN
SGND/PGND
GND
10µF
×2
4.7µF
D3
0.25µH
DCR = 0.32mΩ
VOUT
L4
BG1
LTC3875
TCOMP1
TCOMP2
PHASMD
3.57k
SNSD2+
220nF
220nF
SNSA2+
SNSA1+
RUN1
VOSNS1+
RUN2
VOSNS2+
ITH1
VOSNS1–
ITH2
VOSNS2–
0.1µF
IFAST
TK/SS1
EXTVCC
TK/SS2
PGOOD
FREQ
TRSET2
100k
TAVG
L3
MB3
3.57k
SNSD1+
SNS1–
0.25µH
DCR = 0.32mΩ
0.1µF
ENTEMPB
SNS2–
CLKOUT
100pF
SW1
BG2
1µF
MT3
TG1
BOOST1
SW2
MB4
715Ω
INTVCC
BOOST2
0.1µF
10µF
×2
D4
VIN
TG2
MT4
2.2Ω
220nF
220nF 715Ω
3.01k
ILIM
TRSET1
MODE/PLLIN
SGND/PGND
1k
D1–D4: CMDSH-3
L1–L4: WÜRTH 744301025
MTx: BSC050NE2LS
MBx: BSC010NE2LSI
GND
July 2015 : LT Journal of Analog Innovation | 11
Figure 5. A single-output, 4-phase (1.0V/120A) converter featuring LTC3875 and LTC3874
2.2Ω
10µF
×2
4.7µF
D2
VIN
TG2
MT2
0.25µH
DCR = 0.32mΩ
L2
INTVCC
BOOST2
0.1µF
SW1
SNSD2+
220nF
715Ω
220nF
ENTEMPB
SNS1–
SNSA2+
SNSA1+
RUN2
VOSNS2+
ITH1
VOSNS1–
ITH2
VOSNS2–
0.1µF
TK/SS1
EXTVCC
TK/SS2
PGOOD
220pF
100k
220nF 715Ω
100k
3.01k
ILIM
FREQ
TRSET2
4.02k
13.3k
220nF
IFAST
CLKOUT
2.2nF
TAVG
20k
TRSET1
1k
MODE/PLLIN
SGND/PGND
GND
2.2Ω
10µF
×2
D4
4.7µF
0.25µH
DCR = 0.32mΩ
TG0
BOOST0
0.1µF
L3
MB4
715Ω
220nF
SW1
BG1
ISENSE0
ISENSE1+
ISENSE0–
ISENSE1–
LOWDCR
FAULT1
MODE0
ILIM
MODE1
ITH0
PHASMD
SYNC
12 | July 2015 : LT Journal of Analog Innovation
0.25µH
DCR = 0.32mΩ
L4
MB3
715Ω
220nF
EXTVCC
FAULT0
ITH1
120k
0.1µF
LTC3874
+
1µF
MT3
BOOST1
BG0
RUN1
100pF
INTVCC
TG1
SW0
RUN0
2N7002
10µF
×2
D3
VIN
MT4
+
100µF
6.3V
×14
VOSNS1+
RUN1
10nF
VOUT
1V
120A
3.57k
SNSD1+
SNS2–
L1
MB1
BG1
LTC3875
TCOMP1
TCOMP2
PHASMD
0.25µH
DCR = 0.32mΩ
0.1µF
BG2
3.57k
1µF
MT1
TG1
BOOST1
SW2
MB2
10µF
×2
D1
VIN
4.5V TO 14V
FREQ
GND
75k
GND
D1–D4: CMDSH-3
L1–L4: WÜRTH 744301025
MTx: BSC050NE2LS
MBx: BSC010NE2LSI
330µF
2.5V
×12
design features
The LTC3875 delivers an outsized set of features for its small 6mm × 6mm
40-pin QFN. It offers high efficiency with reliable current mode control, ultralow
DCR sensing and strong integrated drivers. Tracking, multichip operation,
and external sync capability fill out its menu of features.
additional temperature compensation circuit can be used to guarantee
the accurate current limit over a wide
temperature range, and DCR variation.
Efficiency can be optimized with an
ultralow DCR inductor. As shown in
Figure 2, the total solution efficiency in
forced continuous mode (CCM) is 87.1%
at 12V input and 1.0V, 120A output.
The hot spot (bottom MOSFET) temperature rise is 58.1°C with 200 LFM
airflow as shown in Figure 3, where the
ambient temperature is about 25°C.
The DC current sharing among the
four channels is shown in Figure 4. The
difference at full load is about 2.0A
(±3.5%) with a 0.32mΩ DCR inductor.
THE LTC3874 SLAVE CONTROLLER
REDUCES SOLUTION SIZE AND
COMPONENT COUNT IN ALTERNATE
1V, 120A CONVERTER
Figure 5 shows an alternative to the
4.5V~14V input, single-output application shown in Figure 1—in this case using
an LTC3875 and an LTC3874. The LTC3874
phase extender acts as a slave controller, but it supports all the programmable
features as well as fault protection.
• ITH pins of the LTC3875 and LTC3874
are connected for current sharing.
•The CLKOUT pin of the LTC3875 is connected to the SYNC pin of the LTC3874
to synchronize switching frequency.
•The MODE pin of the LTC3874 is
connected to PGOOD, which allows
DCM operation during start-up
period for pre-bias load condition.
•The FAULT pin of the LTC3874 is pulled
up to the INTVCC pin and is connected
to the PGOOD pin of LTC3875 via a
TK/SS pin voltage-controlled MOSFET.
When the PGOOD pin is pulled low due
to a fault, the LTC3874 can shut down
both channels for protection purposes.
CONCLUSION
The LTC3875 delivers an outsized set of
features for its small 6mm × 6mm 40-pin
QFN. It offers high efficiency with reliable current mode control, ultralow DCR
sensing and strong integrated drivers.
Tracking, multichip operation, and external sync capability fill out its menu of
features. Furthermore, the slave controller LTC3874 offers a smaller footprint
solution when paired with the LTC3875.
The LTC3875 and LTC3874 are ideal for
high current applications, such as telecom and datacom systems, industrial
and computer systems applications. n
Like the LTC3875, the LTC3874’s current
mode control is accurate even with
sense signals from an inductor DCR
below 1mΩ. Compared to the master
LTC3875, the LTC3874 simplifies pinout
and uses only one set of RC components
for DCR current sensing. The filter time
constant of the RC filter should have a
bandwidth five times larger than that
of the L/DCR of the output inductor.
The total solution efficiency and thermal performance is similar to that of the
two-LTC3875 solution. The DC current
sharing among four channels is accurate. The difference at full load is about
1.6A with a 0.32m Ω DCR inductor.
July 2015 : LT Journal of Analog Innovation | 13
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