DN528 - Dual Phase Buck Controller Drives High Density 1.2V/60A Supply with Submilliohm DCR Sensing

Dual Phase Buck Controller Drives High Density 1.2V/60A
Supply with Submilliohm DCR Sensing – Design Note 528
Mike Shriver
Introduction
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 LTC®3774
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.
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
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and
PolyPhase are registered trademarks and Hot Swap is a trademark of Linear
Technology Corporation. All other trademarks are the property of their
respective owners.
10k
22pF
VCIN
10k
22µF
INTVCC
4.64k
10k
2.2Ω
+
180µF
LTC3774
0.22µF
SNSD1+
SNS1–
SNSA1+
PGOOD1
PGOOD2
SNSA2+
SNS2–
SNSD2+
0.22µF
COUT1
100µF
4×
0.22µF
ITEMP2
ITH2
VOSNS2–
VOSNS2+
TK/SS2
HIZB2
PWMEN2
PWM2
RUN2
GND
4.7µF
VIN
ILIM1
PHSMD
FREQ
MODE/PLLIN
CLKOUT
INTVCC
VIN
ILIM2
0.22µF
10k
1µF
931Ω
10k
10k
2.2µF
2.2µF
VCIN
INTVCC
5V BIAS
1Ω
VDRV
INTVCC
ITEMP1
ITH1
VOSNS1–
VOSNS1+
TK/SS1
HIZB1
PWMEN1
PWM1
RUN1
GND
10k
37.4k
L1
0.33µH
DISB
22µF
0.22µF
PHASE
VSWH
PWM
BOOT
FDMF6820A
VIN
VIN
22µF
PGND
22µF
CGND
VIN
10k
10k
PHASE
VSWH
4.64k
931Ω
2.2Ω
0.22µF
L2
0.33µH
DN528 F01
DISB
330pF
VIN
SMOD
3.01k
3.3nF
2.2Ω
PWM
BOOT
FDMF6820A
PGND
VIN
7V TO 14V
CGND
RUN1
2.2µF
VDRV
2.2µF
0.01µF
VIN
5V BIAS
1Ω
SMOD
10k
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 0.3µH 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.
L1, L2: WÜRTH 744301033
COUT1: MURATA GRM31CR60J107ME39L
COUT2: SANYO 2R5TPE330M9
Figure 1. Dual Phase, 1.2V/60A LTC3774 Converter Operating at fSW = 400kHz, 7V ≤ VIN ≤ 14V
07/14/528
COUT2
330µF
6×
1.2V/60A
VOUT
20
90
15
POWER STAGE:
DrMOS = FDMF6820A
RBOOST = 2.2Ω
L = WÜRTH 744301033
(0.33µH, 0.32mΩ)
85
80
12
8
75
70
POWER LOSS (W)
EFFICIENCY (%)
95
4
0
10
20
30
40
50
LOAD CURRENT (A)
60
70
0
DN528 F03
DN528 F02
Figure 2. Efficiency and Power Loss Curves for Circuit
Shown in Figure 1. VIN = 12V, VOUT = 1.2V
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 pulseskipping mode or Burst Mode® operation providing
a smooth turn-on into a prebiased output. The PWM
outputs of the LTC3774 can also interface with power
block devices and gate drivers with external MOSFETs.
DCR Sensing
The ultralow DCR sensing capability is a result of an
innovative current sensing technique that improves
the signal-to-noise ratio of the current sense signal.
The external filter tied to the SNSA+ pin amplifies the
AC portion of the DCR-sensed 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 produces less than a
1°C temperature difference between the two phases.
The LTC3774 also 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 I • R drops. The total
Data Sheet Download
www.linear.com/LTC3774
Linear Technology Corporation
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
regulated feedback voltage accuracy is ±0.75% over
temperature. The output voltage range of the LTC3774
is 0.6V to 3.5V.
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 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.
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.
Conclusion
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.
For applications help,
call (408) 432-1900, Ext. 3720
dn528f LT/AP 0714 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 2014
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