DN434 - Multiphase DC/DC Controller Pushes Accuracy and Bandwidth Limits

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Multiphase DC/DC Controller Pushes Accuracy and Bandwidth
Limits
Design Note 434
Tick Houk
In addition to high accuracy, the LTC3811’s low minimum on-time (typically 65ns) allows users to convert a
12V input to a 1V output at switching frequencies up to
750kHz, optimizing load transient response and reducing
the solution size.
Introduction
Speed and accuracy don’t always go hand-in-hand
in DC/DC converter systems—that is, until now. The
LTC3811 is a dual output, fixed frequency current mode
DC/DC switching regulator controller designed for one of
today’s most demanding power supply applications: high
current, low voltage processor core supplies.
A Dual Output, 2-Phase Supply with Differential
Remote Sensing and Inductor DCR Sensing
Figure 1 illustrates a dual output supply using the LTC3811.
The 1.5V, 15A output is regulated using the integrated
differential remote sense amplifier and tracks the output of
channel 1 during start-up. Both outputs use DCR sensing
in order to maximize efficiency and operate 180° out of
With supply current requirements in excess of 100A
and supply voltages as low as 1V, every milliohm of
PCB resistance and every millivolt of IR drop count. The
LTC3811 has an output voltage tolerance of ±0.5% over
temperature, giving power supply designers unprecedented flexibility when making component value and
board layout choices.
, LT, LTC, LTM and PolyPhase are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
1Ω
47μF
X5R
0.1μF
BAT54
RJK0305DPB
EXTVCC VIN INTVCC DRVCC
TG1
0.1μF
SW2
BG1
1.21k
BG2
CLKOUT
PGND
PLL/LPF
PGOOD1
MODE/SYNC
PGOOD2
PHASMODE
0.4μH
0.5mΩ
CS/OUT
RJK0301DPB
100k
SENSE1+
1.21k
680nF
SENSE2+
SENSE1–
SENSE2–
RNG2
DIFF/IN+
RNG1
DIFF/IN–
330μF
×2
47μF
X5R
4.99k
1%
100pF
DIFF/OUT
FB2
FB1
VOUT2
1.5V
15A
33pF
COMP2
COMP1
56k
0.4μH
0.5mΩ
1.21k
7.5k
1%
RUN1
33pF
1.21k
100k
RUN2
11.8k
1%
VOUT1
2V
15A
INTVCC
LTC3811
680nF
RJK0305DPB
0.1μF
BOOST2
SW1
RJK0301DPB
VIN
4.5V TO 14.5V
10μF
X5R
BAT54
TG2
BOOST1
330μF
16V
SS/TRACK1 SGND SS/TRACK2
5nF
100pF
56k
4.99k
1%
47μF
X5R
330μF
×2
DN434 F01
2.43k
1.21k
Figure 1. Dual Output, 2-Phase Supply with Differential Remote Sensing and Inductor DCR Sensing
02/08/434
phase in order to reduce the size of the input capacitor.
Figure 2 illustrates the load step response for channel 2.
Figure 3 illustrates low duty cycle waveforms for a 20V
input, 1.2V output application.
In order to accommodate the use of low DCR inductors
and still maintain good control over the maximum output
current, the current sense voltage for each channel is programmable from 24mV to 85mV using the RNG pins.
For noise-sensitive applications, where the switching
frequency needs to be synchronized to an external clock,
the LTC3811 contains a PLL with an input range of 150kHz
to 900kHz. In addition, the MODE/SYNC, PHASEMODE
and CLKOUT pins allow multiple LTC3811s to be daisychained in order to produce a single high current output.
The LTC3811 can be configured for 2-, 3-, 4-, 6- or 12phase operation, extending the load current range to
beyond 200A.
The LTC3811 has a 4.5V to 30V input voltage range and
is available in two package options: a 38-pin 5mm × 7mm
QFN and a 36-pin SSOP.
A Tried-and-True Architecture
The fixed frequency, peak current mode control architecture was chosen for its excellent channel-to-channel
current matching and its robust cycle-by-cycle current
limit. Current sensing can be done using either a resistor
in series with the inductor or by sensing the DCR of the
inductor with an RC filter. This gives the user a choice
between optimum control of the maximum inductor current and maximum efficiency.
VOUT
50mV/DIV
AC COUPLED
IL
5A/DIV
20μs/DIV
VIN = 12V
VOUT = 1.5V
ILOAD = 0.5A TO 8A
DN434 F02
Figure 2. Load Step Response
Load Step Improvement with Voltage Positioning
For single-output multiphase applications, the LTC3811
contains an amplifier for voltage positioning purposes.
The current sense input voltages are converted to
an output current by a multiple-input, single-output
transconductance amplifier, so that an error voltage
proportional to the load current can be introduced at the
input of the differential amplifier. This transconductance
amplifier allows the user to program an output load
line, improving the DC and AC output accuracy in the
presence of load steps. Figure 4 illustrates the load step
response for a 2-phase, single output power supply using the LTC3811.
Conclusion
The LTC3811 is a versatile, high performance synchronous
buck controller optimized for low voltage, high current
supply applications. With an output accuracy of ±0.5%
and a remote sensing differential amplifier, it represents
a new benchmark for DC/DC converters. It can easily
be configured for either single- or dual-output supplies,
inductor DCR sensing or a sense resistor, and it takes
advantage of Linear Technology’s proprietary PolyPhase®
current sharing architecture. The combination of a very
low minimum on-time and a fixed frequency peak current
mode control architecture no longer force the power supply
designer to trade off performance for protection.
VOUT
50mV/DIV
AC COUPLED
SW
10V/DIV
IL1
5A/DIV
IL
2A/DIV
IL2
5A/DIV
VIN = 20V
VOUT = 1.2V
ILOAD = 1.5A
250ns/DIV
DN434 F03
Figure 3. LTC3811 Low Duty Cycle Waveforms
VIN = 12V
50μs/DIV
VOUT = 1.5V (2-PHASE)
ILOAD = 0A TO 15A
DN434 F04
Figure 4. Load Step Response for a 2-Phase, Single Output
Supply with Voltage Positioning (Forced Continuous)
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2134
www.linear.com
Linear Technology Corporation
dn434f LT/TP 0208 241K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2007
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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