Datasheet
22
LT1725
1725fa
TYPICAL APPLICATIO S
U
Figure 9. 48V to Isolated 15V Converter
R2
0.1Ω
M1
IRF620
T1
VP5-0155
R12
5.1Ω
V
OUT
15V
1725 F09a
R7
51k
R6
51k
C7
47pF
6314
109
8
7
13 12 4 5
15
11 1
2
16
C6
1nF
R8
6.2k
R11
150Ω
D3
1N5257
D4
1N5257
D2
MBRS1100
R10
18Ω
V
IN
R1
24k
C1
22µF
R14
820k
R15
33k
R9
51Ω
R5
51k
R4
3.01k
1%
R3
34.0k
1%
C8
0.1µF
+
C4
150µF
D1
MBRD660
D5
BAS16
+
C9
470pF
C3
100pF
C2
1.5µF
×3
8
5
9
4
1
12
2
11
10
3
6
7
C5
1µF
R13
750Ω
1W
C10
100pF
•
••
•
•
•
PGND
I
SENSE
GATE
V
C
FB
V
CC
UVLO3V
OUT
SGNDR
CMPC
R
OCMP
MENAB
LT1725
ENDLYSFSTOSCAP t
ON
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
TM
#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 3V
OUT
features are unused as shown.