LINEAR TECHNOLOGY LTC3425 Technical data

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4-Phase Monolithic Synchronous Boost Converter Delivers 2.5A
with Output Disconnect in a 5mm × 5mm QFN Package

Design Note 332
David Salerno

Introduction

The LTC®3425 is the industry’s first 4-phase, monolithic
synchronous boost converter. It can start up with as little
as 1V and operate with inputs up to 4.5V. The output
voltage range is 2.4V to 5.25V, making it well suited for
battery-powered applications as well as point-of-load
regulation in low voltage systems. The 4-phase architec­ture allows for an effective switching frequency of up to
8MHz, which in turn reduces output ripple current and
peak inductor current by a factor of four (over an equiva­lent single-phase circuit). This allows the use of small, low
cost, low profile inductors and ceramic capacitors even at
high load currents.

Integrated output disconnect allows V

to go to 0V in

OUT

shutdown while eliminating the high inrush current typi­cal of traditional boost converters during start-up. With

V

IN

2V TO 3V

GND

C
1µF

IN

32

C

SS

0.047µF

1

2

L1

2.7µH

3

4

V

OUT

5

L2

2.7µH

6

7

8

R

FF

C

BULK

: TDK C1608X5R1A105M

C

IN

10k

: TDK C3225X5R0J476M

31 30 29 28 27 26

SHDN SYNCIN

SS

GNDA

GNDA

SWA

V

OUTA

V

OUTB

SWB

GNDB

GNDB
REFEN V

R

SGND FB COMP BURST REFOUT PGOOD

OUTS

9

10 11 12

1M

F2

22pF

C

FF

590k

R

F1

V

INRTILIM

LTC3425

13 14 15 16

R
47k

C

: TDK C2012X5ROJ106M ×4

OUT

L1 TO L4: TDK RLF5018T-2R7M1R8

Figure 1. 2-Cell to 3.3V/2.5A Boost Converter

5A peak current capability and an effective switch R

DS(ON)

of 40mΩ (NMOS) and 50mΩ (PMOS), the LTC3425 is
capable of efficiently delivering 2.5A load current from a

0.8mm maximum profile, 32-pin 5mm × 5mm QFN
package.

Multiple Operating Modes Optimize Performance in
Different Applications

The LTC3425 can be configured for automatic Burst

®

operation, fixed frequency mode with forced

Mode
continuous conduction or fixed frequency mode with
pulse skipping. Programmable automatic Burst Mode
operation is ideal for portable applications, where the
load current can vary over a wide range and efficiency is
paramount. A quiescent current of just 12µA in Burst

, LTC, LT and Burst Mode are registered trademarks of Linear Technology
Corporation.

R

75k

LIM

COM SYNCOUT

GNDD

GNDD

V

V

GNDC

GNDC

C

C

P

15pF

0.022µF

25

SWD

OUTD

OUTC

SWC

B

24

23

L4

2.7µH

22

V

BULK

OUT

3.3V

2.5A

PGOOD

C

OUT

10µF
×4

GND

21

20

L3

2.7µH

19

18

17

R
27k

R1

100k

B

DN332 F01

C
47µF

C
330pF

C

R

15k

T

Z

03/04/332

Mode operation extends battery life during light load
operation. For noise-sensitive applications fixed frequency
mode can be selected, with either forced conduction for
low noise at light load or pulse skipping for improved light
load efficiency. Quiescent current in shutdown is less
than 1µA.

Fault Protection

The LTC3425 includes short-circuit protection, program­mable peak current limit and thermal shutdown.

High Power and High Efficiency in
a Small Package

Figure 1 shows an LTC3425 application using all ceramic
capacitors and low profile inductors to deliver 2.5A load
current at 3.3V from a 2-cell input (V

of 2V to 3V).

IN

Maximum component profile is only 2.5mm. In this
example, the oscillator frequency is programmed for
4MHz (1MHz/phase), resulting in 10mV

output ripple

P-P

at full load (see Figure 2). The burst resistor is selected to
automatically transition from Burst Mode operation to
fixed frequency mode when the load exceeds 100mA for
optimal efficiency (peaking at 96%) over the load range
(see Figure␣ 3). No-load input current is only 39µA at 2.4V.

The current mode architecture results in excellent transient
response during a load step (see Figure 4). The use of a
small feed-forward RC network across the top feedback
resistor provides improved transient response and reduced
output ripple in Burst Mode operation, especially when
using low output capacitor values. The feed-forward capaci­tor also compensates for the effect of stray capacitance at
the FB pin when using large value feedback resistors.

The open-drain PGOOD output goes low when V

11.5% below its regulated value and goes high when V

OUT

is

OUT

is within 9% of its regulated value.

Conclusion

The LTC3425’s multiphase architecture makes it possible
to deliver high power in a small, low profile package with
very low output ripple. It offers features that are de­manded in high performance portable applications, in­cluding output disconnect, automatic Burst Mode and
high efficiency over a wide load range. It also offers
flexible design features, including its programmable soft­start, current limit and oscillator frequency, external com­pensation and multiple available operating modes.

Figure 2. Output Voltage Ripple at 2.5A Load
for Converter of Figure 1

100

VIN = 3V

90
80
70
60

50

40

EFFICIENCY (%)

30
20
10

0

0.1

VIN = 2V

1 10 100 1k 10k

VIN = 2.4V

LOAD CURRENT (mA)

DN332 F03

Figure 3. Efficiency vs Load of the Converter in Figure 1

Data Sheet Download

http://www.linear.com/go/dnLT3425

V

OUT

100mV/DIV

AC COUPLED

I

LOAD

0.5A/DIV

0A

VIN = 2.4V 100µs/DIV DN332 F04
V

= 3.3V

OUT

= 0.5A TO 1.5A

I

LOAD

Figure 4. Response of the Circuit in Figure 1
to a 1A Load Step

Figure 5. The LTC3425 Can Deliver 10W in a
Low Profile 0.7in2 Footprint

For applications help,

call [phone], Ext. 2759

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

dn332f LT/TP 0304 344K • PRINTED IN THE USA

 LINEAR TECHNOLOGY CORPORATION 2004