Dual Phase Buck Controller Drives High Density 1.2V/60A Supply with Sub-Milliohm DCR Sensing

Dual Phase Buck Controller Drives High Density 1.2V/60A
Supply with Sub-Milliohm DCR Sensing
Mike Shriver
Designers of low output voltage rails for communication, networking, server and
industrial systems are challenged to achieve greater load currents and higher
efficiency in diminishing board space. The LTC3774 dual output buck controller
eases this burden by interfacing easily with DrMOS devices, providing high efficiency
and small size by integrating MOSFET and gate driver in the same package. The
LTC3774 can sense current across the inductor’s DCR, with values as low as
0.2mΩ, improving efficiency by eliminating the need for a discrete sense resistor.
The LTC3774’s peak current mode architecture provides cycle-by-cycle current limit,
inherent cycle-by-cycle current sharing and easy to design type II compensation.
HIGH EFFICIENCY CONVERTER WITH
A SMALL FOOTPRINT
Figure 1 shows a dual phase
1.2V/60A LTC3774 converter operating
at a switching frequency of 400kHz.
The power stage for each phase is the
FDMF6820A DrMOS, which comes in
a 6mm × 6mm QFN package, and a
0.3µ H single winding ferrite inductor with
a typical DCR of 0.325mΩ. The resulting
full load efficiency is 89.8%, as shown
in Figure 2. The core of the converter
has a current density of 50A /in2.
DrMOS INTERFACE
The PWM outputs of the LTC3774 are
designed to drive DrMOS devices with a
3-state PWM input. When the PWM signal is high, the top FET is on, and when
the PWM signal is low, the bottom
FET is on. When the PWM signal is floating, both the top FET and bottom FET are
off. This state is used to block inductor reverse current when the LTC3774 is
set up for either pulse-skipping mode
or Burst Mode operation, providing a
smooth turn-on into a prebiased output.
The PWM outputs of the LTC3774 can
32 | July 2014 : LT Journal of Analog Innovation
interface with power block devices and
gate drivers with external MOSFETs.
produces less than a 1°C temperature
difference between the two phases.
DCR SENSING
The LTC3774 provides accurate output
voltage regulation. The output of each
phase is sensed with a differential amplifier placed after the feedback divider to
compensate for any PCB IR drops. The total
regulated feedback voltage accuracy is
±0.75% over temperature. The output voltage range of the LTC3774 is 0.6V to 3.5V.
The ultralow DCR sensing capability is a
result of an innovative current sensing
technique that improves the signal-tonoise ratio of the current sense signal.
The external filter tied to the SNSA+ pin
amplifies the AC portion of the DCRsensed current; the DC current is sensed
via the SNSD+ pin, internally amplified
and summed with the AC portion. This
reconstructed current sense signal seen
by the LTC3774’s current comparator is
effectively amplified by a factor of five,
allowing the converter to remain stable
and retain current limit accuracy for
inductor DCR values as low as 0.2mΩ.
The LTC3774 offers five current limit
settings between 10mV and 30mV with
a worst-case error over temperature
of ±1.25mV. With current mode contol, current sharing between phases
is tightly balanced, as shown by the
thermal image shown in Figure 3. The
1.2V/60A converter operating at full load
PolyPhase OPERATION AND
IMPROVING ROBUSTNESS
The LTC3774 features CLKIN and
CLKOUT pins for PolyPhase operation up
to 12 phases. PolyPhase operation reduces
ripple current for the input capacitors
and in cases where the phases are tied
together, reduces output voltage ripple
and provides faster load step response.
Further improvements in the reliability of single output, redundant (N + 1),
PolyPhase converters can be achieved by
placing Hot Swap™ circuits on the input
and ideal diode circuits on the output of
each phase. If a MOSFET failure occurs, the
fault is isolated and the output is protected
and continues to regulate. Reliability
design ideas
10k
22pF
VCIN
10k
INTVCC
330pF
931Ω
+
ITEMP1
ITH1
VOSNS1–
VOSNS1+
TK/SS1
HIZB1
PWMEN1
PWM1
RUN1
GND
0.22µF
COUT1
100µF
4×
0.22µF
0.22µF
10k
1µF
5V BIAS
1Ω
10k
2.2µF
VIN
22µF
CGND
22µF
Figure 1. Dual phase, 1.2V/60A LTC3774 converter
operating at fSW = 400kHz, 7V ≤ VIN ≤ 14V
PGND
VIN
The LTC3774 is a high performance dual
output buck controller intended for low
output voltage, high output current supplies with DrMOS and ultralow DCR inductors. It yields high efficiency, an accurate
current limit, a precise 0.6V ±0.75%
feedback voltage and fault isolation. n
EFFICIENCY (%)
20
90
15
POWER STAGE:
DrMOS = FDMF6820A
RBOOST = 2.2Ω
L = WÜRTH 744301033
(0.33µH, 0.32mΩ)
80
12
8
75
70
POWER LOSS (W)
CONCLUSION
95
85
L2
0.33µH
L1, L2: WÜRTH 744301033
COUT1: MURATA GRM31CR60J107ME39L
COUT2: SANYO 2R5TPE330M9
10k
Other features include soft recovery
from an output overload, optional
NTC compensated DCR sensing, a
phase-lockable switching frequency
range of 200kHz to 1.2MHz and an
input voltage range of 4.5V to 38V.
COUT2
330µF
6×
931Ω
0.22µF
PHASE
VSWH
10k
is further improved with the LTC3774’s
HIZB pin, which floats the PWM output
when a fault is detected, allowing for a
more predictable shutdown of the DrMOS.
1.2V/60A
VOUT
2.2Ω
PWM
BOOT
FDMF6820A
VIN
4.64k
2.2µF
VCIN
INTVCC
VDRV
2.2Ω
180µF
LTC3774
DISB
VIN
0.22µF
SNSD1+
SNS1–
SNSA1+
PGOOD1
PGOOD2
SNSA2+
SNS2–
SNSD2+
ITEMP2
ITH2
VOSNS2–
VOSNS2+
TK/SS2
HIZB2
PWMEN2
PWM2
RUN2
GND
4.7µF
10k
SMOD
INTVCC
ILIM1
PHSMD
FREQ
MODE/PLLIN
CLKOUT
INTVCC
VIN
ILIM2
L1
0.33µH
4.64k
10k
10k
37.4k
0.22µF
PHASE
VSWH
DISB
22µF
CGND
22µF
PGND
VIN
3.01k
3.3nF
2.2Ω
PWM
BOOT
FDMF6820A
VIN
7V TO 14V
RUN1
2.2µF
VDRV
2.2µF
0.01µF
VIN
5V BIAS
1Ω
SMOD
10k
4
0
10
20
30
40
50
LOAD CURRENT (A)
60
70
0
Figure 2. Efficiency and power loss curves for circuit
shown in Figure 1. VIN = 12V, VOUT = 1.2V
Figure 3. Thermal image of the circuit shown in
Figure 1. fSW = 400kHz, VIN = 12V, VOUT = 1.2V,
IOUT = 60A, no airflow and 21°C ambient
July 2014 : LT Journal of Analog Innovation | 33
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