LT3804

1

3804i

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Final Electrical Specifications

s

Regulates Two Secondary Outputs

s

Optocoupler Feedback Driver and Second Output

Synchronous Driver Controller

s

True Differential Remote Sensing Regulation

s

High Switching Frequency: up to 800kHz

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Programmable Current Limit

s

Programmable Soft-Start and Power Good

s

Automatic Frequency Synchronization

s

Available in Thermally Enhanced 28-Lead TSSOP

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

The LT

regulator with optocoupler feedback control for regulating

multiple outputs in single-secondary winding isolated

power supplies.

The LT3804 contains an error amplifier and an optocoupler

driver to regulate the first (main) output. For the second

output regulation, the LT3804 contains a complete PWM

controller to drive dual synchronous N-channel MOSFETs.

With leading edge modulation, it operates with either

current or voltage mode control of the primary side. The

LT3804 is synchronized to the falling edge of the trans-

former secondary winding and can be used in single-

ended or double-ended isolated power converter topolo-

gies. A user selectable discontinuous conduction mode

improves light load efficiency.

True differential Kelvin sensing is used for each output

feedback amplifier to achieve high regulation accuracy

and design simplicity. Other features include soft start,

current limit and power good flags.

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

s

48V Input Isolated DC/DC Converters

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Multiple Output Power Supplies

s

Offline Converters

s

DC/DC Power Modules

Secondary Side Dual

Output Controller with Opto Driver

June 2003

IN2

IN1

OUT2

OUT1

BIAS

3.3V

AT 15A

BG

SG

LT3781

ISOLATION

BOUNDARY

L1, L2: SUMIDA CEP125-IR8MC-H

Q1-Q4: SILICONIX Si7892DP

36V

TO 72V

TG

•

•

Q1

Q2

L2

1.8

µH

0.003

Ω

•

•

–

+

1.8V

AT 15A

•

•

+

LTC1693-1

CL1N

CL1P

CSET

GNDS1

OPTO

SYNC

TGATE

SW

BGATE

PGND

CL2N

Q4

x2

x2

Q3

CL2P

GNDS2

+

L1

1.8

µH

1.5k

3.01k

604

Ω

LT3804

2.74k

390pF

3804 F01

0.003

Ω

Figure 1. 250kHz, 3.3V and 1.8V Output Isolated DC/DC Converter (Simplified Schematic)