LT1737CS Linear Technology, LT1737CS Datasheet - Page 21

• Manufacturer Part Number: LT1737CS
• Description: IC CTRLR ISOLATED FLYBACK 16SOIC
• Manufacturer: Linear Technology
• Type: Flybackr

Specifications of LT1737CS

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Frequency - Switching: 50kHz ~ 250kHz
• Voltage - Input: 4.1 ~ 20 V
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Voltage - Output: -
• Power - Output: -

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
#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
TYPICAL APPLICATIO S
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
R3
12.7k
1%
R4
3.92k
1%
C5
1nF
25V
X7R
8
7
FB
V
OSCAP
C
6
3V
C6
47pF
50V
NPO
SFST
OUT
9
3
UVLO
t
ON
14
10
R5
51k
5%
8
3
ENDLY
INP
GND
LT1121
13
R6
51k
5%
U2
U
MINENAB
OUT
ADJ
LT1737
12
R7
51k
5%
1
2
Figure 8. 8V-25V to Isolated 5V Converter
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
R
R11
24k
5%
R12
20k
5%
OCMP
4
R8
4.3k
5%
R
CMPC
V
CC
15
5
+
V
C7
0.1µF
25V
Z5U
IN
TM
C3
1µF
25V
Z5U
SGND
C1
330µF
35V
,
11
MBR0540
I
1N5250
SENSE
PGND
GATE
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
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.
1
D2
D3
1737 F08
16
2
5.1Ω
5%
R2
C9
1nF
25V
X7R
R9
68Ω
5%
C4
470pF
50V
X7R
51Ω
R13
5%
CC
6
3
1
4
2
5
•
•
•
supply pin, so a logic level MOSFET has
M1
IRLL014
R1
0.2Ω
0.5W
IRC TYPE LR 2010
T1
COILTRONICS
VP1-0190
•
10
7
•
11
8
•
12
9
MBRD330
D1
OPTIONAL OUTPUT FILTER
+
1µH
L1
C2
68µF
10V
+
LT1737
R10
240Ω
5%
C8
33µF
10V
21
V
5V
500mA
V
5V
500mA
OUT
OUT
1737fa