Datasheet

21
LT1737
1737fa
TYPICAL APPLICATIO S
U
BASIC APPLICATION WITH
3-WINDING TRANSFORMER
Figure 8 shows a compact, low power application of the
LT1737. Transformer T1 is an off-the-shelf VERSA-PAC
TM
,
#VP1-0190, produced by Coiltronics. As manufactured, it
consists of six ideally identical independent windings. In
this application, two windings are stacked in series on the
primary side and three are placed in parallel on the
secondary side. This arrangement provides a 2:1 primary-
to-secondary turns ratio while maximizing overall effi-
ciency. The remaining primary side winding provides a
ground-referred version of the flyback voltage waveform
for the purpose of feedback.
The design accepts an input voltage in the range of 8V to
25V and outputs an isolated 5V. To prevent overvoltage on
the LT1737 and the gate of MOSFET M1, an LT1121 low
dropout linear regulator is employed (U2). Resistor di-
vider R11/R12 sets the output of U2 at nominally 8.25V. (A
few hundred millivolts of dropout will therefore be seen at
the very bottom of the input supply range.) The positive
going drive potential at the LT1737 GATE pin is typically 2V
or so below its V
CC
supply pin, so a logic level MOSFET has
been specified for M1.
Capacitor C6 sets the switching frequency at approxi-
mately 200kHz. Optimal load compensation for the
trans former and secondary circuit components is set by
resistor R8. Resistor R10 provides a guaranteed mini-
mum load of about 20mA to maintain rough output voltage
regulation. The soft-start and UVLO features are unused
as shown.
PGND
I
SENSE
GATE
V
C
FB
R1
0.2Ω
0.5W
IRC TYPE LR 2010
M1
IRLL014
R2
5.1Ω
5%
1 10
7
3
T1
COILTRONICS
VP1-0190
6
4
2
5
R13
51Ω
5%
V
CC
UVLO3V
OUT
SGND
1737 F08
R
CMPC
R
OCMP
MINENAB
LT1737
R7
51k
5%
R6
51k
5%
C6
47pF
50V
NPO
6
8
7
3
910 15
16
2
1
2
8
3
14 13 12 4 5 11 1
ENDLYSFSTOSCAP t
ON
C5
1nF
25V
X7R
R8
4.3k
5%
R9
68Ω
5%
D2
1N5250
D3
MBR0540
R5
51k
5%
R11
24k
5%
C1: SANYO ALUMINUM ELECTROLYTIC (35CV331GX)
C2: SANYO POSCAP (10TPC68M)
C8: SANYO POSCAP (10TPA33M)
D1: MOTOROLA 30V, 3A SCHOTTKY RECTIFIER
D2: 20V, 500mW ZENER DIODE
D3: MOTOROLA 40V, 0.5A SCHOTTKY RECTIFIER
R12
20k
5%
R4
3.92k
1%
R3
12.7k
1%
C3
1µF
25V
Z5U
C1
330µF
35V
V
IN
C7
0.1µF
25V
Z5U
C9
1nF
25V
X7R
C4
470pF
50V
X7R
OUT
U2
LT1121
ADJ
INP
GND
•
•
•
•
11
8
•
12
D1
MBRD330
9
•
+
C2
68µF
10V
+
C8
33µF
10V
L1
1µH
OPTIONAL OUTPUT FILTER
+
R10
240Ω
5%
V
OUT
5V
500mA
V
OUT
5V
500mA
L1: COILCRAFT DO1608C-102 1µH, 0.05Ω INDUCTOR
M1: INT’L RECTIFIER IRLL014 60V, 0.2Ω LOGIC LEVEL N-CH MOSFET
U2: LINEAR TECHNOLOGY MICROPOWER LDO REGULATOR
Figure 8. 8V-25V to Isolated 5V Converter
VERSA-PAC is a trademark of Coiltronics, Inc.