3-Phase Synchronous Step-Down DC/DC Controller with Stage Shedding, Active Voltage Positioning and Nonlinear Control for High Efficiency and Fast Transient Response

3-Phase Synchronous Step-Down DC/DC Controller with
Stage Shedding, Active Voltage Positioning and Nonlinear
Control for High Efficiency and Fast Transient Response
Jian Li and Kerry Holliday
The LTC3829 is a feature-rich single-output 3-phase synchronous buck controller
with on-chip drivers, remote output voltage sensing, inductor DCR temperature
compensation, Stage Shedding™ mode, active voltage positioning (AVP) and nonlinear
control. It is suitable for input from 4.5V to 38V and output from 0.6V up to 5V. The
LTC3829 provides high efficiency, high power density and versatile power solutions for
computers, telecom systems, industrial equipment and DC power distribution systems.
The LTC3829 is available in 38-pin 5mm × 7mm QFN and 38-pin FE packages.
VIN
VIN
DIFFOUT
20.0k
MODE
RUN
40.2k
VOSENSE–
66.5Ω
RUN
MODE
ILIM
ITH
4.7µF
16V
Q1
Q3
D1 CMDSH-3
0.1µF
ISET
TG1
TG1
DIFFN
SW1
DIFFP
BG1
AVP
SW1
180µF
16V
L1
0.33µH
Q4
S1P
S1N
RSENSE1
0.001Ω
100µF
6.3V
X5R
330µF
2.5V
SANYO
×2
+
DIFFOUT
EXTVCC
PGOOD
PGOOD
TG2
BG1
ITEMP
ITEMP
SW2
GND
BG2
10µF
16V
X5R
BOOST2
0.1µF
Q1,Q5,Q9: RJK0305DPB
Q3,Q4,Q7,Q8,Q11,Q12: RJK0330DPB
TG2
SW2
BG2
Q5
D3 CMDSH-3
L2
0.33µH
BOOST3
TK/SS
SENSE3–
0.1µF
SENSE2–
SENSE3+
VOUT
VIN
D2 CMDSH-3
LTC3829
EXTVCC
SENSE1+
VIN
7V TO 14V
+
GND
VIN
DIFFOUT
CSS
0.1µF
180µF
16V
BOOST1
SENSE1–
SENSE2+
0Ω
CLKOUT
+
INTVCC
INTVCC
VFB
13.5k
40.2k
VOSENSE+
PLLIN
CLKOUT
FREQ
PLLIN
47pF
1nF
10µF
16V
X5R
VIN
0.1µF
30.1k
IFAST
100pF
2.2Ω
100k
Q7
TG3
Q8
S2P
S2N
RSENSE2
0.001Ω
VOSENSE+
10Ω
100µF
6.3V
X5R
+
SW3
VOUT
1.5V
330µF 60A
2.5V
SANYO
×2
GND
BG3
VIN
1000pF
1000pF
1000pF
100Ω
10Ω
10µF
16V
X5R
S3N
–
VOSENSE
100Ω
S3P
Figure 1. A 1.5V/60A,
3-phase converter
featuring the LTC3829
100Ω
100Ω
100Ω
100Ω
32 | October 2011 : LT Journal of Analog Innovation
TG3
S2N
Q9
SW3
S2P
S1N
S1P
BG3
Q11
L3
0.33µH
Q12
S3P
S3N
RSENSE3
0.001Ω
100µF
6.3V
X5R
+
330µF
2.5V
SANYO
×2
VOUT
design features
The LTC3829’s constant-frequency peak current mode
control architecture allows a phase-lockable frequency of
up to 770kHz. Even at this high frequency, high step-down
ratios are possible, thanks to the LTC3829’s ability to operate
at low duty cycle due to its small minimum on-time (90ns).
LTC3829 FEATURES
22
The LTC3829 is a current mode PolyPhase®
controller, similar to the LTC3850, but with
an integrated a high speed differential
amplifier for remote output voltage sensing, which can eliminate regulation errors
due to PCB voltage drops at heavy loads.
Figure 1 shows a typical 7V~14V input,
1.5V/60A output application schematic.
20
18
VR(SENSE) (%)
16
Figure 2 shows the tightly balanced
DC current sharing between stages.
Dynamic current sharing is also well balanced cycle-by-cycle due to the LTC3829’s
peak current mode architecture.
Figure 3. Efficiency with and without Stage Shedding
mode enabled
14
12
10
8
6
4
PHASE 1
PHASE 2
PHASE 3
2
0
PolyPhase Operation and High
Step-Down Ratios at High Frequency
The LTC3829’s three channels run
120° out-of-phase, reducing input
RMS current ripple, as well as the required
input capacitance. The CLKOUT and
PLLIN pins enable up to 6-phase operation with multiple LTC3829s.
Accurate DCR Current Sensing
over Temperature
VIN = 12V
VOUT = 1.5V
–2
0
10
20
40
30
ILOAD (mA)
50
60
Figure 2. Current sharing performance
The LTC3829’s constant-frequency
peak current mode control architecture allows a phase-lockable frequency
of up to 770kHz. Even at this high
frequency, high step-down ratios are
possible, thanks to the LTC3829’s ability to operate at low duty cycle due to
its small minimum on-time (90ns).
The LTC3829’s maximum current sense
voltage is selectable—30mV, 50mV or
75mV—allowing the use of either the
inductor DCR or a discrete sense resistor as the current sensing element. The
inductor winding resistance (DCR) changes
over temperature. So to improve accuracy, the LTC3829 can sense the inductor temperature via the ITEMP pin and
maintain a constant current limit over
a broad temperature range. This makes
high efficiency inductor DCR sensing more
reliable for high current applications.
Stage Shedding for Improved
Light Load Efficiency
At heavy loads, the LTC3829 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,
Figure 4. Stage Shedding mode: 3-phase to 1-phase
transition
Figure 5. Stage Shedding mode: 1-phase to 3-phase
transition
95
EFFICIENCY (%)
FORCED
CONTINUOUS
MODE
85
VOUT
100mV/
DIV
VOUT
20mV 100mV/
DIV
VSW1
10V/DIV
VSW1
10V/DIV
VSW2
10V/DIV
VSW2
10V/DIV
VSW3
10V/DIV
VSW3
10V/DIV
30mV
STAGE SHEDDING MODE
80
75
0
10
ILOAD (A)
100
10µs/DIV
10µs/DIV
L = 330nH
QT = RJK0305DPB
VIN = 12V
RSENSE = 1mΩ QB = 2xRJK0330DPB
VOUT = 1.5V
FSW = 400kHz
October 2011 : LT Journal of Analog Innovation | 33
Linear Technology’s proprietary programmable
Stage Shedding feature can further improve the
power supply efficiency in loads up to ~30%. At light
loads, two of the three channels can be shut down
in order to reduce switching-related losses.
VOUT
100mV/DIV
200mV
VOUT
100mV/DIV
125mV
LTC3829
AVP
RPRE-AVP
DIFFP
DIFFN
RAVP
VOUT
ILOAD
20A/DIV
ILOAD
20A/DIV
100µs/DIV
Figure 6. Programmable AVP
with the output capacitors supplying
energy during internal sleep periods. This
provides the highest possible efficiency
at very light load. Forced continuous
mode offers continuous PWM operation
from no load to full load, providing the
lowest possible output voltage ripple.
In addition, Linear Technology’s proprietary programmable Stage Shedding feature
can further improve the power supply
efficiency in loads up to ~30% of full load
as shown in Figure 3. At light loads, two
of the three channels can be shut down in
order to reduce switching-related losses.
When the MODE pin is tied to INTVCC ,
the LTC3829 enters Stage Shedding mode.
This means that the second and third
channels stop switching when the ITH pin
voltage is below a certain programmed
threshold. This threshold voltage,
VSHED, on the ITH pin is programmed
according to the following formula:
VSHED = 0.5 +
5
(0.5 − VISET )
3
34 | October 2011 : LT Journal of Analog Innovation
Figure 7. Transient performance without AVP
Connecting a single resistor from the
ISET pin to SGND sets VISET by way of the
precision 7.5µ A current source from ISET.
Stage Shedding mode in the LTC3829
features smooth transitions when dropping from 3-phase to 1-phase operation and likewise when increasing from
1-phase to 3-phases, with minimum ripple
on the output, as shown in Figures 4
and 5. The smooth transition is a direct
result of current mode control—a voltage mode, multiphase supply would have
trouble achieving this performance.
100µs/DIV
Figure 8. Transient performance with AVP
The AVP scheme modifies the regulated
output voltage depending on its current
loading. The LTC3829 senses inductor
current information by monitoring voltage across the sense resistor, RSENSE or
the DCR sensing network of all three
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:
Active Voltage Positioning ( AVP )
Transient performance is a priority in high
current power supply designs. To minimize
the voltage deviation during load steps, the
LTC3829 includes two features that lower
peak-to-peak output voltage deviation for
a given load step: one is the programmable
active voltage positioning (AVP); the other
is the programmable nonlinear control.
RDROOP = RSENSE •
V
 
RPRE− AVP  A 
R AVP
With proper design, AVP can reduce
the magnitude of transient induced
peak-to-peak voltage spikes by 38%,
as shown in Figures 7 and 8.
design features
The LTC3829 3-phase step-down controller fits an
outsized feature set into a small 5mm × 7mm 38-pin
QFN, making it ideal for high current applications,
including telecom and datacom systems,
industrial and high performance computers.
VOUT
100mV/DIV
95mV
VOUT
100mV/DIV
VSW1
10V/DIV
VSW1
10V/DIV
VSW2
10V/DIV
VSW2
10V/DIV
VSW3
10V/DIV
VSW3
10V/DIV
2µs/DIV
2µs/DIV
Figure 9. Transient performance without nonlinear control
Nonlinear Control
The LTC3829 features a unique nonlinear
control loop that can improve transient
response dramatically. In the nonlinear
control loop, an internal circuit monitors the output of the error amplifier. If
the amplifier is sinking or sourcing large
output currents (level programmable),
the supply output voltage has significant overshoot or undershoot. This is
when nonlinear control takes over: the
controller simultaneously turns all of
the TG signals on at a load step up, or
off at a load step down to avoid control
loop or PWM switching cycle delays.
This feature is enabled and programmed
through the IFAST pin. When the IFAST pin
is tied to INTVCC , the nonlinear control loop is disabled. The IFAST pin
75mV
Figure 10. Transient performance with nonlinear control
sources a precision 10µ A, so connecting a resistor from IFAST to SGND sets
VIFAST. When VIFAST is set below 0.5V,
the difference of 0.5V and VIFAST sets the
threshold voltage that triggers nonlinear control. Nonlinear control is only
enabled when the feedback voltage
VFB is within the UV and OV window.
Once nonlinear control is enabled, the
top gate of all channels is turned on if:
VFB = VREF −
0.5 − VIFAST
• 1.2
5
The top gate of all channels
is turned off if:
VFB = VREF +
0.5 − VIFAST
5
where VREF is the reference voltage (0.6V).
With proper design, nonlinear control
can improve transient response by
21% during the load step up transient,
as shown in Figures 9 and 10.
CONCLUSION
The LTC3829 3-phase step-down controller fits an outsized feature set into a small
5mm × 7mm 38-pin QFN. It offers high
efficiency with strong integrated drivers
and Stage Shedding/Burst Mode operation. It supports temperature compensated
DCR sensing for high reliability. Its AVP and
nonlinear control can improve transient
response with minimum output capacitance. Output tracking, multi-device current sharing and external sync capability
fill out its menu of features. The LTC3829 is
ideal for high current applications, including telecom and datacom systems, industrial and high performance computers. n
October 2011 : LT Journal of Analog Innovation | 35
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