LINEAR TECHNOLOGY LTC3785 Technical data

Buck-Boost Controller Simplifi es Design of DC/DC Converters for
Handheld Products – Design Note 424

David Burgoon

Introduction

A n u m b e r o f c o n v e n t i o n a l s o l u tions have been available for
the design of a DC/ DC converter where the output voltage
is within the input voltage range—a common scenario
in Li-Ion battery-powered applications—but none were
very attractive until now. Conventional topologies, such
as SEPIC or boost followed by buck, have numerous
disadvant ages, including low effi ciency, complex magnet­ics, polarit y inversion and/or circui t complexity/cost. The

®

3785 buck-boost controll er yields a simple, effi cient,

LTC
low parts-count, single-converter solution that is easy
to implement, thus avoiding the drawbacks associated
with traditional solutions.

High Effi ciency Controller Capabilities

The LTC3785 serves applications requiring input and
output voltages in the range of 2.7 to 10V—ideal for
applications powered from one or two Li-Ion cells or
m u l t i p l e c e l l N i M H , N i C a d o r A l k a l i n e b a t t e r i e s . I t s u p p o r t s
a single inductor, 4-switch, buck-boost topology, which is
ideal for an output voltage that is within the input voltage

130k

130k

1nF

2.2nF

21.5k

42.2k

59k

1

3

2

4

5

6

7

8

9

15

13

14

RUN/SS

FB

LTC3785EMS

V

C

V

SENSE

I

LSET

CCM

RT

MODE

SYNC

BG2

SW2

I

SSW2

GND

V

IN

V

CC

I

SVIN

V

BST1

TG1

SW1

I

SSW1

BG1

V

DRV

I

SVOUT

V

BST2

TG2

25

L1: TOKO B1000AS-3RON

V

OUT

215k

215k

range. The high level of integration yields a simple, low
parts-count solution. The LTC3785 provides for current­limit and shutdown in all modes of operation (which is
not normally available with boost converters).

Very high effi ciency is achieved by synchronous recti­fi cation, high side drive (allowing the use of N-channel
MOSFETs), R

current sensing, and Burst Mode®

DS(ON)

operation for effi cient light load operation. Protection
features include soft-start, overvoltage, undervoltage
and foldback current-limit with burp-mode or latch-off
for extended faults.

3.3V, 3A Converter Operates from 2.7V – 10V Source

The circuit shown in Figure 1 utilizes the LTC3785 con­troller to produce a synchronous, 4-switch, buck-boost
design. It provides a fi xed 3.3V, 3A output from a 2.7V
– 10V input. It is short-circuit protected: the controller
offers a choice of recycling or latch-off protection for
severe overload faults.

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

24

23

22

21

20

19

18

17

16

10

11

12

CMDSH-3

0.22μF

CMDSH-3

0.22μF

4.7μF

Q1A

FDS6894A

Q1B

Q2A

FDS6894A

Q2B

DN424 F01

22μF
16V

L1
3μH

V

IN

2.7V TO 10V

47μF

6.3V

V

3.3V
3A

OUT

Figure 1. Schematic of Buck-Boost Converter Using the LTC3785 to Provide 3.3V at 3A from a 2.7V to 10V Source

09/07/424

The circuit produces seamless operation throughout the
entire input voltage range, operating as a synchronous
buck converter, synchronous boost converter, or a com­bination of the two through the transition region. At input
voltages well above the output, the converter operates in
buck mode. Switches Q1A and Q1B commutate the input
voltage, and Q 2A stays on, connect ing L1 to the out put. As
the input volt age is reduced and approaches the regula ted
output voltage, the converter approaches ma ximum duty
cycle on the input (buck) side of the bridge, and the output
(boost) side of the bridge starts to switch, thus entering
the buck-boost or 4-switch region of operation. As the
input is reduced further, the converter enters the boost
region at the minimum boost dut y cycle. Switch Q1A stays
on, connecting the inductor to the input while switches
Q2A and Q2B commutate the output side of the induc­tor between the output capacitor and ground. In boost
mode, this converter has the ability to limit input current,
and also to shut down and disconnect the source from
the output—two desirable features that a conventional
boost converter cannot provide. Figures 2, 3 and 4 show
input side and output side switch waveforms along with
inductor current for buck (10V input), boost (2.7V input)
and buck-boost (3.8V input) modes of operation.

V

SW1

5V/DIV

I

L1

2A/DIV

V

SW2

5V/DIV

1μs/DIV

Figure 2. Input Side and Output Side Switch Waveforms
Along with Inductor Current for Buck (10V Input) Mode
Operation for the Circuit in Figure 1

V

SW1

5V/DIV

I

L1

2A/DIV

V

SW2

5V/DIV

1μs/DIV

Figure 3. Input Side and Output Side Switch Waveforms
Along with Inductor Current for Boost (2.7V Input) Mode
Operation for the Circuit in Figure 1

DN424 F02

DN424 F03

V

SW1

5V/DIV

I

L1

2A/DIV

V

SW2

5V/DIV

1μs/DIV

DN424 F04

Figure 4. Input Side and Output Side Switch Waveforms
Along with Inductor Current for Buck-Boost (3.8V Input)
Mode Operation for the Circuit of Figure 1

95% Effi ciency

Figure 5 shows effi ciency curves for both normal (not
forced continuous c onduction) and Burst Mode operation.
Exceptional e ffi ciency of 95% is achieved at typical loads,
resulting from sophisticated controller features includ­ing high side drivers and R

current sensing. Even

DS(ON)

higher effi ciencies are possible by using a larger ferrite

2

inductor. This circuit easily fi ts in 0.6in

with components
on both sides of the board. The curves show how Burst
Mode operation improves effi ciency at extremely light
loads—an important determinant of battery run time.

Conclusion

The LTC3785 is the latest addition to our class of buck­boost converters developed by Linear Technology to
satisf y the requirements of ba ttery-powere d applications,
specifi call y those requiring an output volt age that is within
the input voltage range. A topology bas ed on the LTC3785
controller overcomes the defi ciencies of conventional
designs. It is elegant in its simplicity, high in effi ciency,
and requires only a small number of inexpensive external
components.

100

90

Burst Mode

OPERATION

80

70

60

50

EFFICIENCY (%)

40

30

20

10

0.001 0.1 1 10

Figure 5. Effi ciency in Normal and Burst Mode Operation
for the Circuit in Figure 1

V

IN

0.01
OUTPUT CURRENT (A)

2.7V

3.0V

4.2V

DN424 F05

Data Sheet Download

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

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call [phone], Ext. 3420

dn424f LT/TP 0907 305K • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2007