LINEAR TECHNOLOGY LTC3856 Technical data
2-Phase Synchronous Buck Controller Features Light Load Stage
Shedding Mode, Active Voltage Positioning, Low R
Remote V
Sensing
OUT
SENSE
Design Note 486
Charlie Zhao and Jian Li
Introduction
Today’s computer, datacom, and telecom systems demand power supplies that are effi cient, 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 effi ciency—especially at heavy load—while
reducing component cost and required board space.
®
The LTC
3856 single-output 2-phase synchronous buck
c o n t r o l l e r i m p r o v e s t h e a c c u r a c y o f i n d u c t o r D C R s e n s i n g
by compensating for changes in DCR due to temperature.
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-
CLKOUT
PLLIN
FREQ
RUN
TK/SS
I
TH
V
FB
AVP
ITEMP
PHASMD
DIFFOUT
DIFFN
DIFFP
ISET
EXTV
CC
ILIM
MODE
PGOOD
2.2Ω
LTC3856
PGND
V
IN
0.1μF
S
SENSE1
V
IN
SENSE1
BOOST1
INTV
BOOST2
SENSE2
SGND
SENSE2
TG1
SW1
BG1
TG2
SW2
BG2
100Ω
100Ω
1nF
–
+
CC
4.7μF
1nF
100Ω
D1, CMDSH-3
RJK0305DPB
D2, CMDSH-3
INTV
0.1μF
100Ω
INTV
CC
+
–
S
20k
PGOOD
1nF
100pF
100k, 1%
S
30.1k
100k
0.1μF
INTV
5.6k
S
CC
S
age sensing, Stage Shedding™ mode for improved light
load effi ciency and adaptive voltage positioning for fast
transient respons e. The LTC3856 can conver t 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 Effi ciency, 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
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.
V
IN
Q1
RJK0305DPB
RJK0305DPB
0.1μF
RJK0330DPB
INTV
Q3
CC
RJK0330DPB
Q2
CC
RJK0305DPB
Q4
Q5
V
IN
Q7
22μF
Q6
RJK0330DPB
22μF
Q8
RJK0330DPB
22μF
L1
0.22μH
22μF
L2
0.22μH
0.001Ω
0.001Ω
S
10Ω 10Ω
100μF
6.3V
s 4
+
+
and
180μF
16V
s 2
330μF
2.5V
4.7μF
s 4
6.3V
V
IN
4.5V
TO 14V
GND
V
OUT
1.5V/
50A
01/11/486
Figure 1. 1.5V/50A, 2-Phase Synchronous Buck Converter Featuring the LTC3856
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 archi tecture, 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 c an 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 INTV
CC
.
The ef fi ciency 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 effi ciency than the LTC3729, a comparable
single-output, 2-phase controller, over the whole load
range. With Stage Shedding mode, signifi cant effi ciency
improvement is further achieved at light load. At 5% load,
the effi ciency is improved by 13%.
Current mode control allows the LTC3856 to transition
smoothly from 2-phase to 1-phase operation and vice
versa.
Active Voltage Positioning
User-selectable act ive 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
temperat ure coeffi cient, causing the e ffective current limit
of the conver ter to change with induc tor temperature. The
LTC3856 can sense th e inductor temperature with an NTC
thermistor, thereby adjusting the current limit based on
the temperat ure. The resul t is a constant current limit over
a broad temperature range. This improves inductor DCR
sensing reliability in high current applications.
V
OUT
50mV/DIV
I
L1
10A/DIV
I
L2
10A/DIV
I
LOAD
20A/DIV
100μs/DIV
dn486 F02
Figure 2. Load Transient and Current Sharing: VIN = 12V,
25A to 50A Load Step
95
1-PHASE
90
85
80
EFFICIENCY (%)
75
13% AT 5% LOAD
70
0
2-PHASE
1.7% AT 20% LOAD
7% AT 10% LOAD
LTC3856, CCM
LTC3856, PHASE SHEDDING
LTC3729, CCM
LOAD CURRENT (A)
5020 4010 30
dn486 F03
Figure 3. Effi ciency Comparison: VIN = 12V, VO = 1.5V,
FSW = 400kHz, L = 220nH, R
SENSE
= 1mΩ,
QT = RJK0305DPB, QB = 2xRJK0330DPB
Data Sheet Download
www.linear.com
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 off set, unity-ga in, high bandwidth dif ferential amplifi er
for true remote sensing. Common mode noise and ground
loop disturbances can be rejected, and load regulation is
greatly improve d, especially wh en 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 contro ller. It achi eves high
effi ciency in both heavy load and light load conditions,
with temperature compensated DCR sensing and Stage
Shedding mode or Burst Mode oper ation. 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 fi ll 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
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
dn486f LT/AP 0111 226K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2010
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