DN486 - 2-Phase Synchronous Buck Controller Features Light Load Stage Shedding Mode, Active Voltage Positioning, Low RSENSE and Remote VOUT Sensing

2-Phase Synchronous Buck Controller Features Light Load Stage
Shedding Mode, Active Voltage Positioning, Low RSENSE and
Remote VOUT Sensing
Design Note 486
Charlie Zhao and Jian Li
Introduction
Today’s computer, datacom, and telecom systems demand power supplies that are efficient, respond quickly
to load transients and accurately regulate the voltage
at the load. For example, load current can be measured
by using the inductor DCR, thus eliminating the need
for a dedicated sense resistor. Inductor DCR sensing
increases efficiency—especially at heavy load—while
reducing component cost and required board space.
The LTC ®3856 single-output 2-phase synchronous buck
controller improves the accuracy of inductor DCR sensing
by compensating for changes in DCR due to temperature.
age sensing, Stage Shedding™ mode for improved light
load efficiency and adaptive voltage positioning for fast
transient response. The LTC3856 can convert a wide input
voltage range, 4.5V to 38V, to outputs from 0.6V to 5V.
Despite the many features, the chip is small, available in
32-pin 5mm × 5mm QFN and 38-pin TSSOP packages.
High Efficiency, 2-Phase, 4.5V to 14V Input,
1.5V/50A Output Converter
Figure 1 shows a typical LTC3856 application in a 4.5V
to 14V input, 1.5V/50A output converter. The LTC3856’s
two channels operate out-of-phase, which reduces the
input RMS current ripple and thus the required input
capacitance. Up to six LTC3856s can be paralleled for
DCR temperature compensation is just one of many
performance enhancing features offered in the LTC3856.
It also includes on-chip gate drivers, remote output volt-
L, LT, LTC, LTM, Linear Technology, the Linear logo and 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.
VIN
1nF
S
0.1μF
2.2Ω
5.6k
100pF
100Ω
Q1
RJK0305DPB
S
S
CLKOUT
100k, 1%
0.1μF
VIN
PLLIN
SENSE1+
FREQ
TG1
RUN
SW1
VFB
S
AVP
30.1k
0.1μF
LTC3856
BOOST1
BG1
INTVCC
D1, CMDSH-3
INTVCC
4.7μF
ITEMP
PHASMD
TG2
DIFFOUT
BOOST2
D2, CMDSH-3
PGOOD
0.1μF
SW2
EXTVCC
BG2
Q7
RJK0305DPB
L2
0.22μH
Q4
RJK0330DPB
0.001Ω
Q8
RJK0330DPB
SENSE2+
MODE
S
22μF
22μF
INTVCC
ISET
ILIM
100μF
6.3V
+
s4
VIN
Q3
RJK0305DPB
1nF
PGOOD PGND SGND SENSE2–
100Ω
100Ω
S
10Ω
10Ω
Figure 1. 1.5V/50A, 2-Phase Synchronous Buck Converter Featuring the LTC3856
01/11/486
180μF
16V
s2
VIN
4.5V
TO 14V
GND
VOUT
1.5V/
50A
0.001Ω
INTVCC
DIFFP
INTVCC
L1
0.22μH
Q6
RJK0330DPB
Q2
RJK0330DPB
DIFFN
100k
22μF
22μF
Q5
RJK0305DPB
1nF
TK/SS
ITH
20k
SENSE1–
+
VIN
100Ω
330μF
2.5V
s4
4.7μF
6.3V
up to 12-phase operation. The LTC3856 has a phaselocked loop (PLL) and can be synchronized to an input
frequency between 250kHz and 770kHz. Due to its peak
current-mode control architecture, the LTC3856 provides
fast cycle-by-cycle dynamic current sharing plus tight DC
current sharing, as shown in Figure 2.
Stage Shedding Mode
At light loads, the LTC3856 can be programmed to operate
in one of three modes: Burst Mode® operation, forced
continuous mode or Stage Shedding mode. With Stage
Shedding mode, the LTC3856 can shut down one channel
to reduce switching related loss which is the dominant
loss at light loads. Stage Shedding mode is selected by
simply tying the MODE pin to INTVCC.
The efficiency improvements achieved by Stage Shedding
mode are shown in Figure 3. Due to strong gate drivers
and shorter dead-time, the LTC3856 can achieve 4% ~
5% higher efficiency than the LTC3729, a comparable
single-output, 2-phase controller, over the whole load
range. With Stage Shedding mode, significant efficiency
improvement is further achieved at light load. At 5% load,
the efficiency is improved by 13%.
VOUT
50mV/DIV
IL1
10A/DIV
IL2
10A/DIV
ILOAD
20A/DIV
dn486 F02
100μs/DIV
Figure 2. Load Transient and Current Sharing: VIN = 12V,
25A to 50A Load Step
95
1-PHASE
2-PHASE
90
EFFICIENCY (%)
1.7% AT 20% LOAD
85
7% AT 10% LOAD
80
LTC3856, CCM
LTC3856, PHASE SHEDDING
LTC3729, CCM
75
13% AT 5% LOAD
70
10
0
20
30
LOAD CURRENT (A)
40
50
dn486 F03
Figure 3. Efficiency Comparison: VIN = 12V, VO = 1.5V,
FSW = 400kHz, L = 220nH, RSENSE = 1mΩ,
QT = RJK0305DPB, QB = 2xRJK0330DPB
Data Sheet Download
www.linear.com
Current mode control allows the LTC3856 to transition
smoothly from 2-phase to 1-phase operation and vice
versa.
Active Voltage Positioning
User-selectable active voltage positioning (AVP) is another
unique design feature of the LTC3856. AVP improves
overall transient response and reduces required output
capacitance by modifying the regulated output voltage
depending on its current loading. With proper design,
AVP can reduce load transient-induced peak-to-peak
voltage spikes by 50%.
Inductor DCR Sensing Temperature Compensation
Although not used here, inductor DCR sensing offers a
lossless method of sensing the load current. The problem
is that the DCR of the inductor typically has a positive
temperature coefficient, causing the effective current limit
of the converter to change with inductor temperature. The
LTC3856 can sense the inductor temperature with an NTC
thermistor, thereby adjusting the current limit based on
the temperature. The result is a constant current limit over
a broad temperature range. This improves inductor DCR
sensing reliability in high current applications.
Output Voltage Remote Sensing
For high output current, low voltage applications, board or
wire interconnect resistance can cause a severe load regulation problem. To solve this problem, the LTC3856 includes a
low offset, unity-gain, high bandwidth differential amplifier
for true remote sensing. Common mode noise and ground
loop disturbances can be rejected, and load regulation is
greatly improved, especially when there are long trace runs
between the load and the converter output.
Conclusion
The LTC3856 is a feature-rich single output, 2-phase
synchronous step-down DC/DC controller. It achieves high
efficiency in both heavy load and light load conditions,
with temperature compensated DCR sensing and Stage
Shedding mode or Burst Mode operation. AVP improves
transient response even when the output capacitance is
reduced. Remote sensing, a tight ±0.75% reference voltage accuracy over temperature, voltage tracking, strong
on-chip drivers, multichip operation and external sync
capability fill out its menu of features. The LTC3856 is
ideal for high current applications and can meet the high
standards of today’s power supplies for telecom and
datacom, industrial and computer applications.
For applications help,
call (408) 432-1900, Ext. 3258
Linear Technology Corporation
dn486f LT/AP 0111 226K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2010
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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