LINEAR TECHNOLOGY LTC3705 Technical data

L DESIGN FEATURES

NDRV

GNDPGNDVSLMT

UVLO

LTC3725

Q1
FDC2512

D1
CMPSH1-4

GATE IS

T2

183,4

5,6

1µF

162k

L1: VISHAY IHLP2525CZER0M01
L2: PULSE PA1294.910

33nF

0.03Ω
1W

HAT2165H

×2

HAT2165H

×2

T1

23.4mm × 20.1mm × 9.4mm
PLANAR

• •

• •

V

CC

33nF

15k

365k

100k

1µF

SSFLT

FB/IN

+

FS/IN

–

V

IN

+

36V

TO

72V
V

IN

–

L1

1µH

470pF

47nF

3.3k

2.2µF

FG SW IS–IS

+

1nF

2.2nF
200V

0.0012Ω
2W

SG VINNDRV

Q2

FCX491A

V

CC

GND PGND

REGSD PHASE

SLP

MODE

PT

+

PT

–

FS

FB

RUN/SS

LTC3706

ITH

2.74k

604Ω

10µF

100µF

6.3V
×2

220µF

6.3V

100k

V

OUT

+

3.3V
30A

V

OUT

–

0.1µF

5.1k

••

L2

0.85µH

1µF
100V

1µF
100V
×2

Si7450DP

5

2
4

3

10

11

7
9

1.2Ω
1/4W

T1: PULSE PA0815 (6:6:2:1)
T2: PULSE PA0297 (2:1:1)

+

VCC, PRI

VCC, SEC

V

GATE

LOAD CURRENT (A)

5

EFFICIENCY (%)

90

48V

72V

36V

92

94

25

88

86

84

10

15

20

30

Isolated Forward Controllers Offer
Buck Simplicity and Performance

Introduction

Buck converter designers have long
benefited from the simplicity, high
efficiency and fast transient response
made possible by the latest buck
controller ICs, which feature synchro­nous rectification and PolyPhase®
operat i o n. Unfortunately, these
same features have been difficult or
impossible to implement in the buck
converter’s close relative, the forward
converter. That is, until now. The
LTC3706/26 secondary-side synchro­nous controller and its companion
smart gate driver, the LTC3705/25,
make it possible to create an isolated
forward converter with the simplicity
and performance of the familiar buck
converter.

The Benefits of Secondary­Side Control Made Accessible

Many isolated supplies place the
controller IC on the input (primary)
side and rely on indirect synchronous

by Charles Hawkes and Arthur Kelley

rectifier timing and optoisolator feed­back to control the output (secondary).
This architecture is commonly known
as primary-side control. By contrast,
secondary-side control places the
controller IC on the secondary side,
and uses a gate-drive transformer
to directly control the primary-side
MOSFETs. This approach eliminates
the need for an optoisolator and
puts the controller where it is really
needed: with the load. This results in
a significantly faster response, taming
large-signal overshoot and reducing
output capacitance requirements.
In addition, secondary-side control
simplifies the design of the loop com­pensation to that of a simple buck
converter.

With the apparent advantages of
secondary-side control, why is it not
used in more isolated applications?
This is primarily because of the need
for a separate bias supply to power

up the controller on the secondary
side, since there is initially no voltage
present there. With the introduction
of the LTC3706/26 and LTC3705/25,
however, this barrier has now been
completely eliminated. All of the com­plex issues associated with start-up
and fault monitoring in a secondary­side control forward converter have

Figure 2. Efficiency of the converter
shown in Figure 1

10

Figure 1. Complete 100W single-switch high efficiency, low cost, minimum part count, isolated
telecom converter. Other output voltages and power levels require only simple component changes.

Linear Technology Magazine • March 2007

DESIGN FEATURES L

NDRV

GND PGND VSLMT

UVLO

BOOST

LTC3705

BAS21

FQT7N10

0.22µF

10µF
25V

CMPSH1-4

1.2Ω

L1

1.2µH

TG TS BG IS

T2

1µF

162k

L1: COILCRAFT SER2010-122
T1: PULSE PA0807
T2: PULSE PA0297

33nF

30mΩ
1W

2mΩ

2W

Si7336ADP

Si7336ADP
×2

T1

••

MURS120

Si7852DP

Si7852DP

MURS120

V

CC

33nF

15k

1%

365k

1%

100k

2.2µF
25V

SS/FLT

FB/IN

+

FS/IN

–

V

IN

–

V

IN

+

330µF

6.3V
×3

2.2µF
16V

680pF

CZT3019

22.6k
1%

20k

102k
1%

V

OUT

–

V

OUT

+

••

1µF
100V
x3

FG SW SG VINNDRV V

CC

GND PGND PHASE SLP MODE REGSD

PT

+

I

S

+

I

S

–

PT

–

RUN/SS

LTC3706

ITH

FB

FS/SYNC

been seamlessly integrated into these
powerful new products. Moreover, a
proprietary scheme is used to mul­tiplex gate drive signals and DC bias
power across the isolation barrier
through a single, tiny pulse transform­er. This eliminates the primary-side
bias winding that is otherwise needed.
The result is an isolated supply that
has been architected from the ground
up to achieve unprecedented simplicity
and performance. Figure 1 illustrates
how this remarkable new architecture
is used to make a complete 100W for­ward converter with minimal design
effort and complexity.

Family of Products Supports
Single or Dual Switch
Topologies

Ta ble 1 s u mm a ri zes how the
LTC3706/26 and LTC3705/25 prod­ucts can be combined to cover a broad
range of applications. The LTC3706
is a full-featured product available
in a 24-lead SSOP package. For high
precision applications, the LTC3706
includes a 1% accuracy output voltage,
a remote-sense differential amplifier
and a power good output voltage moni­tor. The high voltage linear regulator
controller simplifies the design of the
bias supply, and PLL frequency syn­chronization with selectable phase
angle enables PolyPhase operation
with up to twelve phases. In addition,
the flexible current-sense inputs allow

Table 1. LTC3705/06/25/26 combinations

LTC3706 LTC3726

LTC3705

LTC3725

Dual-Switch,

PolyPhase

Single-Switch,

PolyPhase

Dual-Switch,
Single Phase

Single-Switch,

Single Phase

for the use of either resistive or cur­rent transformer sensing techniques.
Protection features include an output
overvoltage crowbar as well as current­limiting and over-current protection.
The 16-lead LTC3726 does not include
the remote voltage sensing or linear
regulator features, so it is more suit­able for a single phase application.
Both the LTC3706 and the LTC3726
have a selectable maximum duty cycle
limit of either 75% or 50% to support a
single or dual-switch forward converter
application, respectively.

The LTC3725 primary driver is
intended for use in single-switch
forward converter. The LTC3725 in­cludes a start-up linear regulator and
an integrated bridge rectifier for bias
generation. Protection features include
volt-second limit, over-current protec­tion and a fault monitoring system that
detects a loss of encoded gate-drive
signal from the signal transformer.
The LTC3705 is a dual-switch forward
driver, and includes an 80V (100V
transient) high side gate driver. The
integration of this high side driver into

the LTC3705 greatly facilitates the use
of the simple and robust dual switch
forward converter topology. Figure 3
shows a typical dual-switch converter
application using the LTC3705 and
the LTC3706.

Table 2 highlights some of the rela­tive merits of using either single or dual
switch forward converter topologies.
In general, for applications that have
a limited input voltage variation, or
where a robust and simple design is
a priority, the dual-switch forward
converter may be preferred. For a wide
input voltage application (greater than
2:1), or whenever a lower cost or size
justifies the complication of the trans­former reset design, a single-switch
forward should be used.

Bringing the Power of
PolyPhase to Isolated Supplies

The LTC3706/26 defies typical forward
converter limits by allowing simple
implementation of a PolyPhase current
share design. PolyPhase operation
allows two or more phase-interleaved
power stages to accurately share the
load. The advantages of PolyPhase
current sharing are numerous, includ­ing much improved efficiency, faster
transient response and reduced input
and output ripple.

The LTC3706/26 supports stan­dard output voltages such as 5V, 12V,
28V and 52V as well as low voltages
down to 0.6V. Figure 4 shows how

11

Linear Technology Magazine • March 2007

Figure 3. Isolated forward converter for 36V–72V input to 3.3V/20A out