LT1425 Linear Technology, LT1425 Datasheet - Page 17

• Manufacturer Part Number: LT1425
• Description: Isolated Flyback Switching Regulator
• Manufacturer: Linear Technology

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-
and 80% (V
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
TYPICAL APPLICATIONS
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
Transformer T1
DALE
LPE-4841-A307
COILTRONICS
CTX02-13483
INPUT
COM
5V
C1
10 F
25V
IN
100k
= 15V). The isolation is ultimately limited
1000pF
C2
10 F
25V
36 H
27 H
LPRI
0.1 F
47pF
RATIO
1:1:1
1:1
22.1k
1%
3.01k
1%
ISOLATION
500VAC
500VAC
U
1
2
3
4
5
6
7
8
GND
NC
R
V
R
SYNC
SGND
GND
FB
C
REF
10.7 11.5 6.3mm
LT1425
14
(L W H)
R
R
OCOMP
CCOMP
14 2.2mm
SHDN
PGND
– 9V Isolated LAN Supply
GND
GND
IN
V
V
SW
IN
= 5V)
16
15
14
13
12
11
10
9
250mA
200mA
I
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
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
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.
OUT
0.1 F
R3
IN
EFFICIENCY
pin, no bootstrap winding is required and normal
76%
70%
C5
D2
D1
C1, C2, C3, C4 = MARCON THCS50E1E106Z CERAMIC
R1
NOT USED
1N5248
2
4
D1
1
3
T1
CAPACITOR, SIZE 1812. [phone]
7
6
MBRS130LT3
MBR0540TL1
REF
NOT USED
D2
pin. The load compensation
C3
10 F
25V
R1, R2
47
75
R2
C4
10 F
25V
C6
C5, C6
330pF
220pF
LT1425
1424/25 TA03
1.8k
17
–9V
13.3k
OUT
COM
5.9k
R3