2-Phase Synchronous Step-Down DC/DC Controller with Programmable Stage Shedding Mode and Active Voltage Positioning for High Efficiency and Fast Transient Response

design features
2-Phase Synchronous Step-Down DC/DC Controller with
Programmable Stage Shedding Mode and Active Voltage
Positioning for High Efficiency and Fast Transient Response
Jian Li and Charlie Zhao
The LTC3856 is a versatile and feature-rich single-output 2-phase synchronous buck
controller with on-chip drivers, remote output voltage sensing, inductor DCR temperature
compensation, Stage Shedding™ mode and active voltage positioning (AVP). It is suitable for
converting inputs of 4.5V–38V to outputs from 0.6V up to 5V. The LTC3856 facilitates the
design of high efficiency, high power density solutions for telecom and datacom systems,
industrial and medical instruments, DC power distribution systems and computer systems.
The controller is available in 32-pin 5mm × 5mm QFN and 38-pin TSSOP packages.
MAJOR FEATURES
making it possible to produce a large stepdown ratio applications in very little space.
The LTC3856’s constant-frequency peak
current-mode control architecture allows
a phase-lockable frequency of up to
770kHz. For high frequency applications,
the LTC3856 can operate at low duty cycles
due to its small minimum on-time (90ns),
drops during heavy load conditions.
Figure 1 shows a typical 4.5V~14V input,
1.5V/50A output application schematic.
The LTC3856 includes a high speed differential amplifier for remote output
voltage sensing, which can eliminate
the regulation error due to PCB voltage
The LTC3856’s two channels operate
anti-phase, which reduces the input
RMS current ripple and thus the input
Figure 1. A 1.5V/50A, 2-phase converter featuring the LTC3856
VIN
+
VIN
1nF
0.1µF
2.2Ω
S
5.6k
100pF
100Ω
VIN
100Ω
Q1
RJK0305DPB
S
S
CLKOUT
100k, 1%
0.1µF
VIN
PLLIN
TG1
RUN
SW1
VFB
S
AVP
30.1k
0.1µF
LTC3856
BOOST1
BG1
INTVCC
PHASMD
TG2
DIFFOUT
BOOST2
D1, CMDSH-3
INTVCC
4.7µF
D2, CMDSH-3
100k
PGOOD
EXTVCC
0.1µF
SW2
BG2
4.7µF
6.3V
22µF
Q7
RJK0305DPB
Q4
RJK0330DPB
L2
0.22µH
0.001Ω
Q8
RJK0330DPB
SENSE2+
MODE
S
330µF
2.5V
×4
22µF
INTVCC
DIFFP
ILIM
100µF
6.3V
+
×4
VIN
Q3
RJK0305DPB
DIFFN
INTVCC
GND
VOUT
1.5V/
50A
0.001Ω
INTVCC
ITEMP
ISET
L1
0.22µH
Q6
RJK0330DPB
Q2
RJK0330DPB
TK/SS
VIN
4.5V
TO 14V
22µF
22µF
Q5
RJK0305DPB
1nF
SENSE1+
FREQ
ITH
20k
SENSE1–
180µF
16V
×2
1nF
PGOOD PGND SGND SENSE2–
100Ω
100Ω
S
10Ω
10Ω
July 2010 : LT Journal of Analog Innovation | 19
At light load, switching-related power losses
dominate the total loss. With Stage Shedding
mode, the LTC3856 can shut down one channel at
light loads to reduce switching related losses.
VOUT
50mV/DIV
VOUT
100mV/DIV
OVERSHOOT
36mV
VOUT
100mV/DIV
UNDERSHOOT
35mV
IL1
10A/DIV
IL2
10A/DIV
50A
ILOAD
20A/DIV
25A
100µs/DIV
VIN = 12V
VOUT = 1.5V
ILOAD = 25A TO 50A
capacitance. Up to six LTC3856s can be
combined for 12-phase operation by using
the CLKOUT, PLLIN and PHASMD pins. Due
to its peak current mode control architecture, the LTC3856 provides fast cycle-bycycle dynamic current sharing plus tight
DC current sharing, as shown in Figure 2.
The LTC3856’s maximum current sense
voltage is selectable for either 30mV,
50mV or 75mV, allowing the use of either
the inductor DCR or a discrete sense
1-PHASE 2-PHASE
90
EFFICIENCY (%)
+7% AT 10% LOAD
+1.7% AT 20% LOAD
85
80
+13% AT 5% LOAD
75
70
VIN = 12V
VOUT = 1.5V
1
VSW1
10V/DIV
VSW2
10V/DIV
VSW2
10V/DIV
VIN = 12V
VOUT = 1.5V
Figure 2. Load transient performance
95
VSW1
10V/DIV
10
LOAD CURRENT (A)
100
LTC3856 Stage Shedding MODE
LTC3856 FORCED CONTINOUS MODE
LTC3729 FORCED CONTINOUS MODE
Figure 5. Efficiency comparison
20 | July 2010 : LT Journal of Analog Innovation
10µs/DIV
Figure 3. Stage Shedding mode:
2-phase to 1-phase transition
resistor as the sensing element. Inductor
winding resistance (DCR) changes over
temperature, so the LTC3856 senses the
inductor temperature via the ITEMP pin
and maintains a constant current limit
over a broad temperature range. It makes
high efficient inductor DCR sensing more
reliable for high current applications.
At heavy load, the LTC3856 operates in
constant frequency PWM mode. At light
loads, it can operate in any of three modes:
Burst Mode® operation, forced continuous mode and Stage Shedding™ mode.
Burst Mode operation switches in pulse
trains of one to several cycles, with the
output capacitors supplying energy during
internal sleep periods. This provides the
highest possible efficiency at very light
load. Forced continuous conduction mode
(CCM) offers continuous PWM operation
from no load to full load, providing the
lowest possible output voltage ripple.
Programmable Stage Shedding mode is
unique to the LTC3856. In Stage Shedding
mode, one channel can be shut down at
VIN = 12V
VOUT = 1.5V
10µs/DIV
Figure 4. Stage Shedding mode:
1-phase to 2-phase transition
light load to reduce switching related
losses, thus improving efficiency in the
load range up to 20% of full load.
The programmable active voltage positioning (AVP) is another unique design feature
of the LTC3856. AVP modifies the regulated
output voltage depending on its current
loading. AVP can improve overall transient
response and save output capacitors.
STAGE SHEDDING MODE
At light load, switching-related power
losses dominate the total loss. With
Stage Shedding mode, the LTC3856 can
shut down one channel at light loads to
reduce switching related losses. When
the MODE pin is tied to INTVCC, the
AVP
RPRE-AVP
LTC3856
DIFFP
DIFFN
RAVP
Figure 6. Programmable AVP
VOUT
design features
The LTC3856’s two channels operate anti-phase, which reduces the input RMS
current ripple and thus the input capacitance. Up to six LTC3856s can be combined
for 12-phase operation. Due to its peak current mode architecture, the LTC3856
provides fast cycle-by-cycle dynamic current sharing, plus tight DC current sharing.
108mV
VOUT
50mV/DIV
RDROOP
2.1mΩ
VOUT
50mV/DIV
50A
IL
20A/DIV
25A
VIN = 12V
VOUT = 1.5V
50A
IL
20A/DIV
25A
100µs/DIV
Figure 7. Transient performance without AVP
LTC3856 enters Stage Shedding mode.
This means that the second channel stops
switching when ITH is below a certain
programmed threshold. The threshold
voltage VSHED on ITH is programmed
according to the following formula:
VSHED = 0.5 +
54mV
5
(0.5 − VISET )
3
There is a precision 7.5µA flowing out
of the ISET pin. Connecting a resistor to SGND sets the VISET voltage.
Current mode control allows the LTC3856
to transition smoothly from 2-phase to
1-phase operation and vice versa, as shown
in Figures 3 and 4. A voltage mode, multi­
phase supply cannot transition between
1- and 2-phase operation as smoothly.
The efficiency improvements brought
on by Stage Shedding mode are shown
in Figure 5. Due to stronger gate driver
and shorter dead-time, the LTC3856 can
achieve around 4%~5% higher efficiency
than the LTC3729, a comparable singleoutput, 2-phase controller, over the
whole load range. With Stage Shedding
VIN = 12V
VOUT = 1.5V
100µs/DIV
Figure 8. Transient performance with AVP
mode, significant efficiency improvement
is further achieved at light load. At 5%
load, the efficiency is improved by 13%.
ACTIVE VOLTAGE POSITIONING
Transient performance is an important
parameter in the requirements for the
latest power supplies. To minimize the
voltage excursions during a load step,
the LTC3856 uses AVP to lower peak-topeak output voltage deviations caused
by load steps without having to increase
the output filter capacitance. Likewise,
the output filter capacitance can be
reduced in applications while maintaining peak-to-peak transient response.
The LTC3856 senses inductor current
information by monitoring the voltage
across the sense resisters RSENSE or the
DCR sensing network of the two channels.
The voltage drops are added together
and applied as VPRE-AVP between the
AVP and DIFFP pins, which are connected
through resistor RPRE-AVP. Then, VPRE-AVP is
scaled through RAVP and added to the
output voltage as the compensation
for the load voltage drop. As shown in
Figure 6, the load slope (RDROOP) is set to:
RDROOP =
RSENSE • R AVP V
•
RPRE - AVP
A
With proper design, AVP can reduce load
transient-induced peak-to-peak voltage
spikes by 50%, as shown in Figures 7 and 8.
CONCLUSION
The LTC3856 delivers an outsized set of
features for its small 5mm × 5mm 32-pin
QFN package. It can run at high efficiency using temperature compensated
DCR sensing with Stage Shedding mode/
Burst Mode operation. AVP can improve
the transient response even as output
capacitance is reduced. 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, such as
telecom and datacom systems, industrial
and computer systems applications. n
July 2010 : LT Journal of Analog Innovation | 21
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