LINEAR TECHNOLOGY LTC3811 Technical data

Multiphase DC/DC Controller Pushes Accuracy and Bandwidth
Limits

Design Note 434

Tick Houk

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, fi xed 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.

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 fl exibility when making component value and
board layout choices.

0.1μF

BAT54

V

OUT1

15A

1.21k

56k

EXTV

TG1

BOOST1

SW1

BG1

CLKOUT

PLL/LPF

MODE/SYNC

PHASMODE

SENSE1

SENSE1

RNG2

RNG1

RUN2

RUN1

FB1

COMP1

SS/TRACK1 SS/TRACK2SGND

RJK0305DPB

RJK0301DPB

1.21k 1.21k

0.5mΩ

2V

330μF

47μF

×2

X5R

11.8k
1%

4.99k
1%

0.1μF

680nF 680nF0.4μH

33pF

100pF

INTV

V

IN

CC

LTC3811

+

–

5nF

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.

A Dual Output, 2-Phase Supply with Differential
Remote Sensing and Inductor DCR Sensing

Figure 1 illustr ates a dual output supply using the LTC3811.
The 1.5V, 15A output is regulated using the integrated
diff erential remote sense ampli fi er and tracks t he output of
channel 1 during star t-up. Both outputs use DCR sensing
in order to maximize effi ciency and operate 180° out of

, LT, LTC, LTM and PolyPhase are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.

V

IN

47μF
X5R

330μF
16V

4.5V TO 14.5V

330μF
×2

DN434 F01

V

1.5V
15A

OUT2

1Ω

DRV

CC

CC

TG2

BOOST2

SW2

BG2

PGND

PGOOD1

PGOOD2

CS/OUT

SENSE2

SENSE2

DIFF/IN

DIFF/IN

DIFF/OUT

FB2

COMP2

2.43k 1.21k

47μF

10μF
X5R

BAT54

0.1μF

100k

INTV

CC

100k

+

1.21k

–

+

–

7.5k
1%

33pF

100pF

56k

4.99k
1%

X5R

RJK0305DPB

RJK0301DPB

0.4μH

0.5mΩ

02/08/434

Figure 1. Dual Output, 2-Phase Supply with Differential Remote Sensing and Inductor DCR Sensing

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 r ange 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 confi gured for 2-, 3-, 4-, 6- or 12­phase operation, extending the load current range to
beyond 200A.

A Tried-and-True Architecture

The fi xed 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 fi lter. This gives the user a choice
between optimum control of the maximum inductor cur­rent and maximum effi ciency.

V

OUT

50mV/DIV

AC COUPLED

I

L

5A/DIV

VIN = 12V

= 1.5V

V

OUT

= 0.5A TO 8A

I

LOAD

20μs/DIV

Figure 2. Load Step Response

DN434 F02

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.

Load Step Improvement with Voltage Positioning

For single-output multiphase applications, the LTC3811
contains an amplifi er for voltage positioning purposes.
The current sense input voltages are converted to
an output current by a multiple-input, single-output
transconductance amplifi er, so that an error voltage
proportional to the load current can be introduced at the
input of the differential amplifi er. This transconductance
amplifi er 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 vers atile, high performan ce synchronous
buck controller optimized for low voltage, high current
supply applications. With an output accuracy of ±0.5%
and a remote sensing differential amplifi er, it represents
a new benchmark for DC/DC converters. It can easily
be confi gured for either single- or dual-output supplies,
inductor DCR sensing or a sense resistor, and it takes
advantage o f Linear Technology’s propriet ary PolyPhase

®

current sharing architecture. The combination of a very
low minimum on-time and a fi xed frequency peak current
mode control archi tecture no longer force the power supply
designer to trade off performance for protection.

SW

10V/DIV

I

L

2A/DIV

VIN = 20V
V

= 1.2V

OUT

= 1.5A

I

LOAD

250ns/DIV

DN434 F03

Figure 3. LTC3811 Low Duty Cycle Waveforms

Data Sheet Download

www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

V

OUT

50mV/DIV

AC COUPLED

I

L1

5A/DIV

I

L2

5A/DIV

VIN = 12V

= 1.5V (2-PHASE)

V

OUT

= 0A TO 15A

I

LOAD

50μs/DIV

DN434 F04

Figure 4. Load Step Response for a 2-Phase, Single Output

Supply with Voltage Positioning (Forced Continuous)

For applications help,

call [phone], Ext. 2134

dn434f LT/TP 0208 241K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2007