LINEAR TECHNOLOGY LT3759 Technical data

Wide Input Voltage Range Boost/Inverting/SEPIC Controller
Works Down to an Input Voltage of 1.6V

Design Note 500

Zhongming Ye

Introduction

Many of today’s electronic devices require an inverting
or noninverting converter or some times both. They also
need to operate from a variety of power sources includ­ing USB, wall adapters, alkaline and lithium batteries.
To produce various polarity outputs from variable input
voltages, power supply designers often use a variety
of regulator ICs, which makes for a long inventory list.

®

The LT

3759 operates over an input voltage range from

1.6V to 42V and controls either positive or negative
outputs using the same feedback pin, thus shortening
the inventory list and simplifying design. It also packs
many popular features such as soft-start, adjustable
frequency and synchronization into a small footprint.
The LT3759 comes in a 5mm × 4mm 12-pin MSE pack­age and can be used in multiple configurations such as
boost, SEPIC, flyback and Cuk topologies.

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

V

IN

2.5V TO 15V

22µF

C

×4

IN

348k

EN_UVLO

887k

SYNC

27.4k
300kHz

0.1µF

M1: BSC027N04LS
L1, L2: XAL-1010-682-EMB
D1: PMEG4030

GATE

SENSE

FBX

100k

0.02Ω

CC

4.7µF
10V

X5R

V

DRIVE

IN

PGOOD

LT3759

RT

SS

GND

V

C

INTV

1.4k

22nF

I

L1

M1

1.8mΩ

0.5W

C

DC

47µF ×4

16V

L2

L2I

L1

V

SW

Wide Input Voltage Range with Internal LDO

The LT3759’s wide input range simplifies the design of
power supplies that must be c ompatible with a wide arra y
of power sources. There is no need to add an external
regulator or a slow-charge hysteretic start scheme
because the LT3759 includes two internal low dropout
(LDO) voltage regulators powered from V

and DRIVE

IN

respectively, allowing simple start-up and biasing. The
LT3759’s internal INTV

current limit funct ion protect s

CC

the IC from excessive on-chip power dissipation.

Sensing Output Voltage Made Easier

LT3759 features a novel FBX pin archit ecture that simpli­fies the desi gn of inverting and noninver ting converter s.
It contains two internal error amplifiers—one senses
positive outputs and the other negative—allowing the
FBX pin to be connected directly to a divider from either
a positive output or a negative output, eliminating any
confusion associated with positive or negative output
sensing and simplifying the board layout. Simply de­cide the output polarity and topology and the LT3759
does the rest.

100

VIN = 12V

90
80

D1

102k
1%

15.8k
1%

DN500 F01

V

OUT

12V, 2.5A

C

OUT1

47µF
×4

70
60
50
40

EFFICIENCY (%)

30
20
10

0

0

0.5 1

1.5 2.5

LOAD CURRENT (A)

Figure 2. Efficiency for the
Converter in Figure 1

2

DN500 F02

Figure 1. SEPIC Converter Produces 12V Output from 2.5V to 15V Inputs

03/12/500

Adjustable/Synchronizable Switching Frequency

It is often necessary to operate a converter at a par­ticular frequency, especially if the converter is used
in an RF communications product that is sensitive to
spectral noise in certain frequency bands. Also, if the
area available for a converter is limited, operating at
higher frequencies allows the use of smaller compo­nent sizes, reducing the real estate required and the
output ripple. If power loss is a concern, switching at
a lower frequency reduces switching losses, improving
efficiency. The switching frequency can be set from
100kHz to 1MHz via a single resistor from the RT pin
to ground. The device can also be synchronized to an
external clock via the SYNC pin.

Precision UVLO and Soft-Start

Input supply undervoltage (UVLO) for sequencing or
start-up overcurrent protection is easily achieved by
driving the UVLO with a resistor divider from the V

IN

supply. The divider output produces 1.22V at the UVLO
pin when V

is at the desired UVLO rising threshold

IN

voltage. T he UVLO pin has an adjustabl e input hysteresis,
which allows the IC to ignore a settable input supply
droop before disabling the converter. During a UVLO
event, the IC is disabled and V

quiescent current

IN

drops to 1µA or lower.
The SS pin provides access to the soft-start feature,

which reduces the peak input current and prevents
output voltage overshoot during start-up or recovery
from a fault condition. The SS pin reduces the inrush
current by lowering the switch peak current. In this
way soft-start allows the output capacitor to charge
gradually toward its final value.

I

L1 + IL2

10A/DIV

V

SW

20V/DIV

V

OUT

10V/DIV

20µs/DIV

Figure 3. Short-Circuit Event
for the Converter in Figure 1

DN500 F03

V

IN

1.8V TO 4.5V

R

16.9k
500kHz

M1: Si7858ADP
L1: WÜRTH ELEKTRONIK 7443552200
D1: SBR3U20SA

A 2.5V to 15V to 12V SEPIC Converter

Figure 1 shows a 2.5V to 15V input, 12V/2.5A output
SEPIC power supply using the LT3759. The typical ef­ficiency for this converter is shown in Figure 2. Figure 3
shows the switch waveform during an output short­circuit event. Notice how the switching frequency folds
back to one-third of the regular frequency as soon as
the output voltage is shorted to ground. This feature
enhances short-circuit performance of both Cuk and
SEPIC converters.

A 1.8V to 4.5V to 5V/2A Boost Converter

Figure 4 shows a 5V, 2A output converter that takes an
input of 4.5V to as low as 1.8V. The LT3759 is config­ured as a boost converter for this application, where
the converter output voltage is higher than the input
voltage. The 500kHz operating frequency allows the
use of small inductor and output capacitors.

Conclusion

The LT3759 is a versatile IC that integrates a rich set
of unique features in a tiny 5mm × 4mm 12-pin MSE
package. It accepts wide input voltage range from

1.6V to 42V, with low shutdown current and frequency
foldback at output short-circuit. The LT3759 is ideal
for wide input voltage applications from single-cell,
lithium-ion powered systems to automotive, industrial
and telecommunications power supplies. The high level
of integration yields a simple, low parts-count solution
for boost, SEPIC and Inverting converters.

C

IN

R3

47µF

6.3V
X5R

T

C

SS

0.1µF

59k

R4
124k

C

C2

100pF

V

IN

EN_UVLO

SYNC

RT

SS

V

C

LT3759

R

C

7.5k
C
22nF

DRIVE

PGOOD

SENSE

GND

INTV

C1

Figure 4. Boost Converter Produces 5V/2A

Output from 1.8V to 4.5V Input

GATE

FBX

R5

L1

10k

2.2µH
D1

M1

CC

R

S

C

VCC

4.7µF
10V

X5R

5mΩ

0.5W

R2
34k
1%

R1

15.8k
1%

V
5V
2A

C
100µF

6.3V
X5R
×2

DN500 F04

OUT

OUT2

Data Sheet Download

www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

For applications help,

call [phone], Ext. 3798

dn500fa LT/AP 0312 196K • PRINTED IN THE USA

 LINEAR TECHNOLOGY CORPORATION 2012