LT1725IGN datasheet(22/28 Pages) LINER

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LT1725

1725f

TYPICAL APPLICATIO S

Figure 9. 48V to Isolated 15V Converter

0.1

Ω

IRF620

VP5-0155

R12

5.1

Ω

VOUT

15V

1725 F09a

51k

47pF

1nF

6.2k

R11

150

Ω

1N5257

MBRS1100

R10

Ω

VIN

24k

µF

R14

820k

R15

33k

Ω

51k

3.01k

34.0k

0.1

µF

150

µF

MBRD660

BAS16

470pF

100pF

1.5

µF

×3

µF

R13

750

Ω

C10

100pF

•

••

•

PGND

ISENSE

GATE

VCC

UVLO

3VOUT

SGND

RCMPC

ROCMP

MENAB

LT1725

ENDLY

SFST

OSCAP

tON

VERSA-PAK is a trademark of Coiltronics, Inc

TELECOM 48V TO ISOLATED 15V APPLICATION

The design in Figure 9 accepts an input voltage in the

range of 36V to 72V and outputs an isolated 15V at up to

2A. Transformer T1 is an off-the-shelf VERSA-PAK

#VP5-0155, produced by Coiltronics. As manufactured, it

consists of six ideally identical independent windings. In

this application, three windings are stacked in series on

the primary side and two are placed in parallel on the

secondary side. This arrangement provides a 3:1 primary-

to-secondary turns ratio while maximizing overall effi-

ciency. The remaining winding provides a primary-side

ground-referred version of the flyback voltage waveform

for both feedback information and providing power to the

LT1725 itself.

Capacitor C7 sets the switching frequency at approxi-

mately 200kHz. Optimal load compensation for the trans-

former and secondary circuit components is set by resistor

R8. Output voltage regulation and overall efficiency are

shown in the accompanying graphs. The resistor divider

formed by R14 and R15 sets the undervoltage lockout

threshold at about 32V, with a hysteresis band of about 2V.

The soft-start and 3VOUT features are unused as shown.

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