17
LT1425
TYPICAL APPLICATIO
N
S
U
The following are several application examples of the
LT1425. The first shows an isolated LAN supply which
provides – 9V with
±
1% load regulation for output cur-
rents of 0mA to 250mA. An alternate transformer, the
Coiltronics part, provides a complete PCMCIA Type II
height solution. The LT1425 offers excellent load regula-
tion and fast dynamic response not found in similar
isolated flyback schemes.
The next example shows a
±
15V supply with 1.5kV of
isolation. The sum of line/load/cross regulation is better
than
±
3%. Full load efficiency is between 72% (V
IN
= 5V)
and 80% (V
IN
= 15V). The isolation is ultimately limited
only by bobbin selection and transformer construction.
The “– 48V to 5V Isolated Telecom Supply” uses an
external cascoded 200V MOSFET to extend the LT1425’s
35V maximum switch voltage limit. The input voltage
range (– 36V to – 72V) also exceeds the LT1425’s 20V
maximum input voltage, so a bootstrap winding is used.
D1, D2, Q2 and Q3 and associated components for the
necessary start-up circuitry with hysteresis. When C1
charges to 15V, switching begins and the bootstrap wind-
ing begins to supply power before C1 has a chance to
discharge to 11V. Feedback voltage is fed directly through
a resistor divider to the R
REF
pin. The load compensation
circuitry is bypassed, resulting in
±
5% load regulation.
Finally, the “12V to 5V Isolated Converter” is similar to the
previous example in that a cascoded MOSFET is used to
prevent voltage breakdown of the output switch. But
because the nominal 12V input is well within the range of
the V
IN
pin, no bootstrap winding is required and normal
load compensation function is provided. Diode D1, tran-
sistor Q1 and associated components provide an under-
voltage lockout function via the SHDN pin. The off-the-
shelf transformer provides up to 5W of isolated regulated
power.
– 9V Isolated LAN Supply
Transformer T1
LPRI
RATIO
ISOLATION
(L
×
W
×
H)
I
OUT
EFFICIENCY
D1
D2
R1, R2
C5, C6
R3
DALE
LPE-4841-A307
36
µ
H
1:1:1
500VAC
10.7
×
11.5
×
6.3mm
250mA
76%
NOT USED
NOT USED
47
Ω
330pF
13.3k
COILTRONICS
CTX02-13483
27
µ
H
1:1
500VAC
14
×
14
×
2.2mm
200mA
70%
1N5248
MBR0540TL1
75
Ω
220pF
5.9k
1424/25 TA03
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
GND
NC
R
FB
V
C
R
REF
SYNC
SGND
GND
3.01k
1%
R3
R1
R2
2
4
1
3
7
MBRS130LT3
T1
6
22.1k
1%
0.1
µ
F
LT1425
47pF
C5
C6
C3
10
µ
F
25V
C4
10
µ
F
25V
1.8k
OUT
COM
–9V
1000pF
C1
10
µ
F
25V
5V
INPUT
COM
C2
10
µ
F
25V
0.1
µ
F
100k
GND
SHDN
R
OCOMP
R
CCOMP
V
IN
V
SW
PGND
GND
C1, C2, C3, C4 = MARCON THCS50E1E106Z CERAMIC
CAPACITOR, SIZE 1812. (847) 696-2000
D1
D2
