1

LT1681

1681f

APPLICATIO S

DESCRIPTIO

FEATURES

TYPICAL APPLICATIO

Dual Transistor

Synchronous Forward Controller

The LT

synchronous dual transistor forward DC/DC converters. The

part employs fixed frequency current mode control and

supports both isolated and nonisolated topologies. The IC

drivesexternalN-channelpowerMOSFETsandoperateswith

input voltages up to 72V.

The LT1681’s operating frequency is programmable and can

be synchronized up to 350kHz. Switch phase is also con-

trolled during synchronized operation to accommodate mul-

tiple converter systems. Internal logic guarantees 50% maxi-

mum duty cycle operation to prevent transformer saturation.

The LT1681 incorporates a soft-start feature that provides a

controlled increase in supplied current during start-up and

after an undervoltage lockout or overvoltage/overcurrent

event.

The part is available in a 20-lead wide SO package to support

high voltage pin-to-pin clearance.

s

Isolated Telecommunication Systems

s

Personal Computers and Peripherals

s

Lead Acid Battery Backup Systems

s

Automotive and Heavy Equipment

s

High Voltage: Operation Up to 72V

s

Synchronizable Operating Frequency and Output

Switch Phase for Multiple Controller Systems

s

Fixed Frequency Operation to 350kHz

s

Adaptive and Adjustable Blanking

s

Synchronous Rectifier Driver

s

Local 1% Voltage Reference

s

Undervoltage Lockout Protection with Hysteresis

s

Input Overvoltage Protection

s

Programmable Start Inhibit

s

Transformer Primary Saturation Protection

s

Optocoupler Feedback Support

s

Soft-Start Control

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

36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)

14

2

1

5

1

µF

150pF

OVLO

SHDN

73.2k

1%

20k

10k

270k

0.25W

68

µF

20V

6

4.7nF

8

SS

10

15

BAT54

ZVN3310F

9

PGND

37

4

THERM

LT1681

SYNC SGND

52.3k

100

Ω

1k

1%

100

Ω

2k

FZT690

4.7

µF

16V

0.22

µF

50V

CMPZ5242B

12V

3.3

Ω

10k

0.01

µF

13

SG

+

0.1

µF

11

SENSE

16

BG

18

BSTREF

19

TG

BLKSENS

20

BAS21

0.1

µF

100V

C4

1.5

µF

100V

C3

1.5

µF

100V

L1

4.7

µH

0.025

Ω

1/2W

Q3

1nF

100V

1nF

100V

Q5

Q6

MURS120T3

MURS120T3

Q1

+

–

C2

22

µF

100V

+

–

C5

330

µF

10V

10

Ω

0.25W

10

Ω

0.25W

L2

4.1

µH

330pF

+

1.24k

1%

IN2

IN1

GND2

OUT2

OUT1

GND1

8

3

1

4

6

5

7

2

LTC1693-2

1OV

BIAS

1OV

BIAS

MBR-

0540T1

MMBD914LT1

5V

OUT

3.01k

1%

51

Ω

0.047

µF

6

7

5

8

4

9

1

2

3

10

11

12

1nF

4.7

Ω

+

1681 TA01

56k

12

17

CMPZ5248B

18V

CMPZ-

5248B

15V

Q10

24k

T1

C2:SANYO 100MV22AX

C3, C4: VITRAMON VJ1825Y155MXB

C5: 4X KEMET T510X337KO10AS

L1: COILCRAFT DO1608C-472

L2: PANASONIC ETQP6F4R1LF4

Q1,Q3:100V SILICONIX SUD40N10-25

Q5,Q6: SILICONIX Si4450

T1:COILTRONICS VP5-1200

Q10: ON SEMI MMBT3906LTI