Datasheet LTC3705 Datasheet (LINEAR TECHNOLOGY)

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

L DESIGN FEATURES

SSP

V

IN

+

V

IN

–

V

IN

+

V

IN

–

V

OUT

+

1.2V/100A
V

OUT

–

V

OUT

+

V

OUT

–

SYNC

LTC3705/LTC3706

36V-72VIN TO 1.2V

OUT

50A SUPPLY

ITH SSSV

BIAS

SSP

V

IN

+

V

IN

–

V

OUT

+

V

OUT

–

SYNC

LTC3705/LTC3706

36V-72VIN TO 1.2V

OUT

50A SUPPLY

ITH SSSV

BIAS

I

LOUT1

I

LOUT2

10A/DIV

10µs/DIV

V

OUT

0.5V/DIV
2ms/DIV

SECONDARY-SIDE MODE

PRIMARY-

SIDE MODE

V

IN

V

CCPRI

V

CCPRI

SUPPLIED BY Q1

V

GATE

CONTROLLED BY LTC3706

V

GATE

CONTROLLED BY LTC3725

V

CCPRI

SUPPLIED BY

TRANSFORMER T2

V

GATE

V

OUT

V

CC,SEC

V

PT+,VPT–

easy it is to parallel two 1.2V supplies
to achieve a 100A supply. Figure 5
shows excellent output inductor cur­rent tracking during a 0A to 100A load
current step and the smooth handoff
during start-up to secondary-side con­trol at approximately V

= 0.25V.

OUT

Anatomy of a Start-Up:
A Simple Isolated 3.3V,
30A Forward Converter

The circuit of Figure 1 shows a
complete 100W, one-switch forward
converter. In this example, the
LTC3706 controller is used on the
secondary and the LTC3725 driver
with self-starting capability is used
on the primary. This design features
off-the-shelf magnetics and high ef­ficiency (see Figure 2). The start-up
behavior of this supply is illustrated
in Figure 6. When input voltage is
first applied, the LTC3725 uses Q1
to generate a bias voltage V
begins a controlled soft-start of the
output voltage. As the output voltage
begins to rise, the LTC3706 second­ary controller is quickly powered up
by using T1, D1 and Q2 to generate
V
V

. As shown in Figure 6, the

CC,SEC

voltage rises very quickly as

CC,SEC

compared with the output voltage
V

of the converter. The LTC3706

OUT

CC,PRI

, and

Table 2. Single and dual switch forward converter relative merits

Requirement Single-Switch Dual-Switch

Simple Design

Requires Design

Transformer Reset Circuit

to Prevent Saturation

Wide Input Supply Range

(>2:1)

High Efficiency

Low Switch Voltage Stress

Low Cost

Small Size

75% Max Duty

Good

Can be 2 × VIN or Greater

One FET

One FET and Better

Transformer Utilization

then assumes control of the output
voltage by sending encoded PWM gate
pulses to the LTC3725 primary driver
via signal transformer T2. As soon as
the LTC3725 begins decoding these
PWM gate pulses, it shuts down the
linear regulator by tying NDRV to VCC
and begins extracting bias power for
V

from the signal transformer T2.

CC,PRI

This complete transition from primary
to secondary control occurs seamlessly
at a fraction of the output voltage. From

–

+

Reset Circuit not

Required—Can’t Saturate

+

–

50% Max Duty

+

+

Good

–

+

+

Limited to V

–

IN

Two FETs

+

–

Two FETs and 50%

Transformer Utilization

that point on, operation and design
simplifies to that of a simple buck
converter. Even the design and optimi­zation of the feedback loop makes use
of the familiar and proven OPTI-LOOP®
compensation techniques.

A 10V–30V Input, 15V Output
at 5A Forward Converter

Figure 7 highlights the flexibility of
the LTC3706 and LTC3725 by illus­trating a 12V/24V input application.

Figure 4. Paralleling supplies for higher power operation

12

Figure 5. 1.2V, 100A load current step
(top trace) and start-up (bottom trace)

Figure 6. Anatomy of a start-up

Linear Technology Magazine • March 2007

DESIGN FEATURES L

NDRV

GND PGND VSLMT

UVLO

GATE

LTC3725

2.2nF

250VAC

C3

2.2nF
100V

220pF

200V

68pF

L1

13µH

10Ω

0.5W

150Ω

174Ω

IS

T2

1µF

68pF

68pF

162k

470pF

L1: PULSE PA1961.133
T1: PULSE PA0810
T2: PULSE PA0297

C1: NIPPON CHIMICON EMZA500ADA221MUA0G
C2: TAIYO YUDEN GMK325BJ106MN
C3: TAIYO YUDEN TMK325BJ106MM
C4: SANYO OSCON 16SVP180MX

Q1: FMMT38C
Q2: MMBFJ201
Q3: ZVN3320F
Q4: FDMS2572 ×2

Q5: FMMT 618
Q6: FMMT 718
Q7: MMBT 2907A

33nF

5mΩ
2W

6mΩ
1W

Si7852DP

Q7

Q5

Q4

Q6

T1

1:3

2:1

V

CC

68nF

383k

75k

Q1

R1

10k

1µF
25V

SS/FLT

FB/IN

+

FS/IN

–

V

IN

–

V

IN

+

C4
180µF
16V

C6
10µF
200V

C3

10µF

25V

×2

10µF
16V

R2

8.66k

1nF

470pF

FG SW SG VINNDRV V

CC

GND PGND PHASE SLP MODE REGSD

PT

+

I

S

+

I

S

–

PT

–

RUN/SS

LTC3706

ITH

100k

43.2k

1.07k
1%

25.5k
1%

V

OUT

–

V

OUT

+

330pF

33pF

D2 BAT54

C5

0.1µF

R3

33k

0.5W

D1
ES1C

FCX1051A

FB

FS/SYNC

0.1µF

1nF

••

C1
220µF
50V
×2

C2
10µF
35V
×5

100Ω100Ω

100Ω

5.1kΩ

301Ω

IRF6648
×2

Q3

100Ω

Q2

LOAD CURRENT (A)

0

85

90

95

4

2 6

75

80

EFFICIENCY (%)

VIN = 12V

VIN = 24V

V

OUT

200mV/DIV

I

OUT

5A/DIV

20µs/DIV

VIN = 12V
V

OUT

= 15V

LOAD STEP = 0A TO 5A

Figure 7. Isolated forward converter for 10V–30V input to 15V/5A out

In this circuit, the main transformer
T1 is used to step up the voltage so
that the output can be either higher
or lower than the input. This circuit
is an excellent alternative to a flyback
converter where higher efficiency or
lower noise is a priority.

The UVLO on the LTC3725 has been
set to turn on at V
V

= 7.5V, and a linear regulator (Q1)

IN

= 9.5V and off at

IN

is used to establish bias for start-up.
Note that the LTC3725 requires that
the NDRV pin be at least 1V above
the VCC pin for proper linear regulator
operation. To meet this requirement,
while providing the lowest possibly
dropout voltage, a darlington transis­tor is used (Q1). JFET Q2 is used to
provide adequate bias current for the
NDRV pin at low input voltage, while
limiting the maximum current seen at
high input voltage. R11 is needed to
prevent back-feeding of current from
the NDRV pin into base of Q1 (and

Linear Technology Magazine • March 2007

gate of Q2) during normal operation
when V
less than 12V.

On the secondary side, the output
voltage is used directly as a source
of bias voltage for the LTC3706. This
is possible for output voltages of 9V
or greater. Q3 is used to limit the

CC

= V

= 12V and VIN is

NDRV

peak voltage seen by the SW pin on

Figure 8. Transient response
of the circuit in Figure 7

the LTC3706, while still allowing the
detection circuits in the LTC3706 to
function normally. Capacitor C3 is
used to establish the resonant reset
of the main transformer T1 during the
off-time of the primary-side switches.
In order to reduce the inrush current
during start-up, D2, R2 and C5 are

continued on page 39

Figure 9. Efficiency of
the circuit in Figure 7

13

DESIGN IDEAS L

R1

200Ω

R3

200Ω

R4
200Ω

R2

200Ω

5V

PECL LEVELS

1/4 MC10H350
PECL-TTL TRANSLATOR

TTL OUTPUT

OUT

OPTION

700mV

PP

(DIFFERENTIAL)

MINIMUM

(INPUT

CIRCUITRY

OMITTED)

5V

OV

DD

OGND

+

–

LT6411

5V5V

LT1715

20µs/DIV

V

OUT

50mV/DIV

INDUCTOR

CURRENT

5A/DIV

Single-Ended Output

The LT6411 produces a differential
output, but if a single-ended logic
output is needed, there are multiple

options for data conversion. One such
way is shown in Figure 8, in which
the MC10H350 PECL-TTL translator
performs the conversion. To translate

the voltage levels from the LT6411 to
PECL input voltage levels, two resis­tive dividers level-shift and attenuate
the output signal of the LT6411. Al­ternatively, a high speed comparator
such as Linear Technology’s LT1715
can also perform this task without the
level-shifting resistors.

Conclusion

The LT6411 is a dual high speed ampli­fier with flexible features and superb
AC characteristics, making it suitable
for use as a high data rate receiver.
The ability to select different gain
configurations with minimal external
components makes the LT6411 easy
to use. Its small footprint and low
power consumption allow it to fit into

Figure 8. If a single-ended output is needed, there are many options available for translators.
One example is ON Semiconductor’s MC10H350 PECL-TTL translator. The 200Ω resistors shift
the output of the LT6411 up to PECL voltage levels. Alternatively, a level-translating comparator
such as the LT1715 could be used to give a variety of logic output levels.

almost any application without painful
compromises, especially for portable
or peripheral applications where space
and power are at a premium.

L

LT3740, continued from page 36

The LT3740 uses a valley mode cur­rent control system that boasts a fast
response to load changes. As shown
in Figure 3, this design responds to
0A–10A step load change in 10µs,
yielding a voltage transient of less
than 50mV.

Soft-Start

The LT3740 is also equipped with a
flexible soft-start design that allows for
either ramped current or tracking. If
the XREF pin is held above 1V, and an
RC timer is applied to the SHDN pin,
the converter soft-starts by ramping
the current available to the load. If the
SHDN pin is high, enabling the chip,
and a 0V to 0.8V tracking signal is
applied to the XREF pin, the internal
reference of the LT3740 follows the
tracking signal.

LTC3706/26, continued from page 13

used to provide a gradual increase
in peak current during the soft-start
interval. The circuit of Figure 7 also
includes an optional falling-edge delay
circuit on the gate of synchronous
switch Q4. This delay has been used
to optimize the dead time for this
specific application, thereby improving

Linear Technology Magazine • March 2007

Conclusion

The LT3740 is a synchronous buck
controller that boasts a rich feature set
which allows the designer to optimize
power and volumetric efficiency by ex­ploiting the advantages of a low input
voltage. Through a combination of its

Figure 3. Output voltage and inductor current response to a
0A–10A step load transient applied to the circuit in Figure 1

the efficiency by about 1%. Figure 8
shows the transient response that is
achieved using the circuit of Figure 7,
and Figure 9 shows the efficiency at
V

IN

Conclusion

The new LTC3706/26 controller and
LTC3705/25 driver bring an un-

= 12V and V

= 24V.

IN

onboard boost regulator, user pro­grammable current limit thresholds,
fast transient response and flexible
soft-start system, the designer can
produce a small, efficient, full featured
converter.

L

precedented level of simplicity and
performance to the design of isolated
power supplies. Each controller-driver
pair works in concert to offer high
efficiency, low cost solutions using
off-the-shelf components. The devices
are versatile and easy to use, covering
a broad range of forward converter
applications.

L

3939