LT1737IGN(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT1737IGN Datasheet PDF : 28 Pages
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LT1737
TYPICAL APPLICATIO S
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-PACTM,
#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 VCC 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
transformer 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.
VERSA-PAC is a trademark of Coiltronics, Inc.
R3
12.7k
1%
R4
3.92k
1%
6
T1
COILTRONICS
VIN
3 • VP1-0190
D1
MBRD330
VOUT
8 INP OUT 1
U2
R11
3
LT1121
GND ADJ
2
24k
5%
R12
20k
5%
9
10
+ C1
330µF
35V
D2
1N5250
C3
D3
1µF MBR0540
25V
Z5U
15
R9 1 •
68Ω
5%
4
C4 2 •
470pF
50V
X7R 5
R2
10 11 12
•7 •8 •9
5V
+ C2
R10 500mA
68µF 240Ω
10V 5%
L1
1µH
VOUT
5V
3VOUT UVLO
8
VCC
5.1Ω
16 5%
FB
GATE
7
VC
LT1737
ISENSE 2
C5 OSCAP SFST tON ENDLY MINENAB ROCMP RCMPC
1nF
6
3 14 13
12
4
5
SGND PGND
11 1
C9 R13
1nF 51Ω
25V 5%
25V
X7R
C6
47pF
50V
NPO
R5 R6
R7
R8
51k 51k 51k 4.3k
5% 5% 5% 5%
C7
0.1µF
25V
Z5U
X7R
1737 F08
M1
IRLL014
R1
0.2Ω
0.5W
IRC TYPE LR 2010
+ C8 500mA
33µF
10V
OPTIONAL OUTPUT FILTER
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
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
1737fa
21
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