LINEAR TECHNOLOGY LT3710 Technical data

FEATURES

LT3710

Secondary Side

Synchronous Post Regulator

U

DESCRIPTIO

■

Generates a Regulated Auxiliary Output in Isolated
DC/DC Converters

■

0.8V ±1.5% Accurate Voltage Reference

■

Dual N-Channel MOSFET Synchronous Drivers

■

High Switching Frequency: Up to 500kHz

■

Programmable Current Limit Protection

■

Programmable Soft-Start

■

Automatic Frequency Synchronization

■

Small 16-Pin Thermally Enhanced TSSOP Package

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APPLICATIO S

■

48V Isolated DC/DC Converters

■

Multiple Output Supplies

■

Offline Converters

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TYPICAL APPLICATIO

The LT®3710 is a high efficiency step-down switching
regulator intended for auxiliary outputs in single second­ary winding, multiple output power supplies.

The LT3710 drives dual synchronous N-channel MOSFETs
and achieves high efficiency. With leading edge modula­tion, it operates well with either primary side peak current
or voltage mode control. It is synchronized to the falling
edge of the transformer secondary winding and can be
used in both single-ended and double-ended isolated
power converter topologies. A high speed operational
amplifier is incorporated to achieve optimum compensa­tion and fast transient response. A user selectable discon­tinuous conduction mode improves light load efficiency.

The LT3710 is available in a thermally enhanced TSSOP-16
exposed pad power package.

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

36V

TO 72V

+

220Ω

V

OUT1

3.3V
AT 10A

V

OUT2

1.8V
AT 10A

C

OUT2

3710 F01

L1

V

IN

••

BG

TG

V

C

LT3781

SG

V

REF

+

V

FB

–

ISOLATION

BOUNDARY

••

V

CC

BIAS

V

DD

V

COMP

CG

V

FB

FG

I

SNS

LTC1698

SYNC

OPTODRV

GND

: POSCAP, 680µF/4V

C

OUT2

L2: SUMIDA CEP125-IR8MC-H
Q1, Q2: SILICONIX Si7440DP

180pF

10k

10pF

680pF

10k

V

BOOST

CC

SYNC

LT3710

CSET

C

S

ILCOMP
SS

0.01µF

BGS

PGND

CL–CL

PLEASE REFER TO FIGURE 3
IN THE APPLICATIONS SECTION
FOR THE COMPLETE SCHEMATIC

GBIAS

SW

VA

OUT

V

CMDSH-3

4.7µF

L2

TG

0.1µF

BG

3.3k

33nF

FB

+

Q1

1.8µH

Q2 B340A

0.006Ω

4700pF

3.01k

2.32k

Figure 1. Simplified Single Secondary Winding 3.3V and 1.8V Output Isolated DC/DC Converter

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1

LT3710

1
2
3
4
5
6
7
8

TOP VIEW

FE PACKAGE

16-LEAD PLASTIC TSSOP

16
15
14
13
12
11
10

9

BOOST

TGATE

SW

CSET

SYNC

ILCOMP

SS

V

FB

GBIAS
BGATE
PGND
V

CC

CL

–

CL

+

VA

OUT

BGS

17

WW

W

ABSOLUTE AXI U RATI GS

U

UUW

PACKAGE/ORDER I FOR ATIO

(Note 1)

VCC Supply Voltage.................................................. 26V

BOOST Pin Voltage With Respect to SW pin ........... 10V

BOOST Pin Voltage With Respect to GND pin.......... 35V

SYNC Pin Voltage .................................................... 30V

ORDER PART

NUMBER

LT3710EFE

Operating Junction Temperature Range

(Notes 2, 3) ...................................... – 40°C to 125°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec)..................300°C

Note: If higher than 30V on SYNC pin is needed, add a 10k resistor in series with the pin.

FE PART

MARKING

3710EFE

T

= 125°C, θJA = 38°C/W

JMAX

EXPOSED PAD IS SGND (PIN 17) MUST BE

CONNECTED TO PGND AND SOLDERED TO PCB

Consult LTC Marketing for parts specified with wider operating temperature ranges.

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VCC = 11V, operating maximum VCC = 24V, no load on any outputs
unless otherwise noted.

The ● denotes the specifications which apply over the full operating

PARAMETER CONDITIONS MIN TYP MAX UNITS
Overall

Supply Voltage (VCC) ● 824V
Supply Current (I
Boost Pin Current V

) VA

VCC

≤ 1.2V (Switching Off) 7 12 mA

OUT

= VSW + 8V, 0V ≤ VSW ≤ 24V

BOOST

TGATE High 2 3 mA
TGATE Low 2 3 mA

Voltage Amplifier VA

Reference Voltage (V

FB Pin Input Current VFB = V
VA

High 4.5 V

OUT

VA

Low 0.8 V

OUT

VA

Source Current ● 100 300 µA

OUT

) 0.788 0.8 0.812 V

REF

REF

● 0.780 0.820 V

0.2 0.5 µA

Open-Loop Gain 100 dB

Gain Bandwidth Product 10 MHz
Soft-Start Current 51218 µA

Current Limit Amplifier CA1

Current Limit Threshold at (V
BGATE Off Threshold at (V
Switching Off Threshold at ILCOMP V
Input Current (CL+, CL–)V

+

–

– V

CL

+

– V

CL

) Common Mode Voltage from 0V to VCC – 2.5V ● 50 70 85 mV

CL

–

), BGS Pin Float Common Mode Voltage from 0V to VCC – 2.5V 0 8 15 mV

CL

ILCOMP

+

= V

CL

–

CL

100 µA

0.15 V

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2

LT3710

ELECTRICAL CHARACTERISTICS

The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 11V, operating maximum VCC = 24V, no load on any outputs
unless otherwise noted.

PARAMETER CONDITIONS MIN TYP MAX UNITS
Oscillator

Switching Frequency CS = 500pF (No SYNC) ● 170 200 240 kHz

C

= 333pF (No SYNC) ● 240 280 340 kHz

S

Synchronization Frequency Range CS = 500pF ● 245 400 kHz

C

= 333pF ● 345 500 kHz

S

CSET Ramp Valley Voltage CS = 1000pF (No SYNC) 0.90 1.15 1.4 V
CSET Peak-to-Peak Voltage CS = 1000pF (No SYNC) 2.4 V
Synchronization Pulse Threshold on SYNC Pin Falling Edge V
Maximum Duty Cycle VFB = V

SYNC

– 5mV, CS > 500pF ● 85 90 %

REF

Gate Drivers (TGATE, BGATE)

V

GBIAS

V

High (V

TGATE

V

High I

BGATE

V

Low (V

TGATE

V

Low I

BGATE

– VSW)I

TGATE

– VSW)I

TGATE

I

< 25mA ● 7.5 8.0 8.5 V

GBIAS

< 50mA, V

TGATE

< 50mA ● 5 6 7.5 V

BGATE

< –50mA ● 0.5 V

TGATE

< –50mA ● 0.5 V

BGATE

BOOST

= V

– 0.5V ● 567 V

GBIAS

Peak Gate Drive Current 10nF Load 1 A
Gate Drive Rise and Fall Time 1nF Load 25 ns

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LT3710E is guaranteed to meet performance specifications
from 0°C to 125°C. Specifications over the –40°C to 125°C operating
temperature range are assured by design, characterization and correlation

Note 3: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.

with statistical process controls.

2.5 V

UW

TYPICAL PERFOR A CE CHARACTERISTICS

V

vs I

GBIAS

Temperature ICC vs VCC (Switching Off)

8.1

8.0

(V)

7.9

GBIAS

V

7.8

7.7
0

over Junction

GBIAS

–40°C

25°C

125°C

10 20 26

I

(mA)

GBIAS

3710 G01

13

TA = 25°C

12

11

10

9

(mA)

CC

I

8

7

6

5

8 1012141618202224

VCC (V)

3710 G02

Voltage Amplifier VA Gain and
Phase

120

TA = 25°C

80

40

GAIN (dB)

0

–20

10

GAIN

0dB, 10MHz

100 1k 10k 100k 1M 10M 100M

FREQUENCY (Hz)

(–111°)

PHASE

–0

–50

PHASE (DEG)

–100

–150

–180

3710 G03

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3

LT3710

JUNCTION TEMPERATURE (°C)

–20–40 2507550 125

V

REF

(V)

0.801

0.800

0.799

0.798

3710 G05

CSET = 500pF

UW

TYPICAL PERFOR A CE CHARACTERISTICS

∆V

vs VCC, ∆FREQ vs V

(mV)

REF

∆V

–1

REF

CSET = 500pF

= 25°C

T

A

3

2

1

0

∆V

REF

∆FREQ

10 15 20 25

VCC (V)

CC

3710 G04

∆FREQ (kHz)

1

0

–1

V

vs Temperature

REF

Frequency vs Temperature

195

CSET = 500pF

200

205

210

SWITCHING FREQUENCY (kHz)

215

300

250

200

150

100

(mA)

GBIAS

I

–20–40 2507550 125

JUNCTION TEMPERATURE (°C)

G

vs I

BIAS

C

50

0

0

GBIAS

GBIAS

= 2.2µF

3710 G06

(Charging 2.2µF)

V

GBIAS

I

GBIAS

500µs

TIME

3710 G08

1ms

CSET vs Switching Frequency

500

TA = 25°C

CSET

400

300

FREQUENCY (kHz)

200

100

Current Limit Amplifier CA1 Gain
at VCC = 11V, V

8

VCC = 11V

= 5V

V

CLN

(V)

OUT

VA

7

= 25°C

T

A

6

5

4

3

2

1

0

12

10

8

V

GBIAS

6

(V)

4

2

0

MAXIMUM DUTY CYCLE

400200 600 800 1000

CSET (pF)

–

= 5V

CL

CSET PEAK

CSET VALLEY

6050 70 80 90

+

V

–

– V

(mV)

CL

CL

3710 G07

3710 G09

1.00

MAXIMUM DUTY CYCLE

0.95

0.90

0.85

0.80

0.75

0.70

4

3710f