Datasheet

24
LT1725
1725fa
TYPICAL APPLICATIO S
U
TELECOM 48V TO ISOLATED 5V APPLICATION
The design in Figure 10 accepts an input voltage in the
range of 36V to 72V and outputs an isolated 5V at up to 2A.
Transformer T1 is available as a Coiltronics CTX02-14989.
Capacitor C7 sets the switching frequency at approxi-
mately 275kHz. 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. Efficiency is shown
both with and without the R11 preload. The resistor divider
formed by R13 and R14 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.
R2
0.18Ω
M1
IRF620
T1
CTX02-14989
V
OUT
5V
1725 F10a
R7
51k
R6
51k
C7
47pF
6314
109
8
7
13 12 4 5
15
11 1
2
16
C6
1nF
R8
2.7k
R12
68Ω
R1
47k
V
IN
R10
22Ω
R13
820k
R14
33k
R5
51k
R4
3.01k
1%
R3
35.7k
1%
C8
0.1µF
+
C3
150µF
D1
12CWQ06
R9
18Ω
C5
470pF
D2
BAS16
+
C4
150pF
C2
1.5µF
C1
15µF
4
2
11
9
12
10
1
6
R11
51Ω
1W
C9
100pF
C10
1µF
PGND
I
SENSE
GATE
V
C
FB
V
CC
UVLO3V
OUT
SGNDR
CMPC
R
OCMP
MENAB
LT1725
ENDLYSFSTOSCAP t
ON
Figure 10. 48V to Isolated 5V Converter
I
LOAD
(A)
0
V
OUT
(V)
5.00
1725 F10b
4.75
0.5
1.0
1.5
2.0
5.25
V
IN
= 36V
V
IN
= 72V
V
IN
= 48V
I
LOAD
(A)
0.01
60
EFFICIENCY (%)
70
80
90
0.1 1 10
1725 F10c
50
40
30
20
V
IN
= 48V
WITHOUT R11
PRELOAD
WITH R11
PRELOAD
Application Regulation Application Efficiency