LT1737CS Linear Technology, LT1737CS Datasheet - Page 24

• 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: -

LT1737
R3 and D2. (Diode D2 prevents reverse emitter/base
breakdown in Q1 when MOSFET M1 is in the “ON” state.)
The raw flyback voltage at the drain of MOSFET M1 minus
the V
to a voltage at R3. Or, stated mathematically:
Resistor R13 provides an initial pre-load to the supply
output to improve light load regulation. Resistor divider
R8/R9 sets the undervoltage lockout threshold at nomi-
nally 10.4V for turn-on, with turn-off about 600mV lower.
Overall power supply efficiency and output regulation
versus input voltage and load current may be seen in
Figures 13 and 14.
TYPICAL APPLICATIO S
2N3906
24
Q1
V
C1A-B, C2A-B: SANYO POSCAP (10TPB220M)
D1: MOTOROLA 35V, 8A SCHOTTKY DIODE
D2: SIGNAL DIODE
D3: MOTOROLA 20V, 0.5A SCHOTTKY DIODE
D4: 10V, 500mW ZENER DIODE
M1, M2: SILICONIX/VISHAY 25V, 0.023Ω N-CH MOSFET
R1: 5 × 0.10Ω, 1W (IRC LR2512)
T1: COILTRONICS TRANSFORMER
FB
BE
R3
3.01k
1%
=
of Q1 is converted to a current by R2 and then back
(
V
FLBK
C3
1nF
25V
X7R
D2
BAS16
8
7
FB
V
OSCAP
–
C
V
6
11.0k
BE
1%
3V
R2
C5
47pF
50V
NPO
SFST
OUT
)
9
⎛
⎜
⎝
3
R
R
UVLO
t
3
2
ON
⎞
⎟
⎠
14
10
R4
75k
5%
ENDLY
13
R5
51k
5%
U
MINENAB
LT1737
12
R6
51k
5%
Figure 15. 4.5V-5.5V to Isolated 5V Converter
R
OCMP
V
CC
15
4
R7
2.2k
5%
R
CMPC
C8
1µF
25V
Z5U
5
C4
0.1µF
25V
Z5U
MBR0520
SGND
1N5240
220µF
11
C1A
10V
D4
D3
I
SENSE
PGND
GATE
+
5V
The LT1737 is a bipolar technology IC specified to operate
down to a minimum input supply voltage of 4.5V. Although
its GATE pin drives “low” nearly to ground, its “high”
capability is limited by a headroom requirement of roughly
2V
the GATE output will only drive up to a nominal 3V or so.
Fortunately, MOSFETs are now available with specified
performance at this level of gate voltage.
The circuit shown in Figure 15 provides an isolated 5V
output from an input between 4.5V and 5.5V. Two Si9804
low gate voltage MOSFETs are paralleled to handle the
primary-side current—up to 12A peak. This circuit pro-
vides more output power than the previous examples. It
1
IN
BE
16
2
220µF
s. Thus when operating at a worst case 4.5V supply,
C1B
10V
APPLICATION
R8
10Ω
5%
C9
1nF
25V
X7R
C6
4.7nF
50V
X7R
+
12
1
V
51Ω
•
R11
IN
5%
M1
SI9804
11
2
•
COILTRONICS
10
3
VP5-0083
•
T1
R1
0.02Ω
1737 F15
•
4
9
•
M2
SI9804
5
8
•
6
7
R9
15Ω
5%
MBRD835L
C7
2.2nF
50V
X7R
D1
+
220µF
C2B
10V
C2A
220µF
10V
+
R10
270Ω
5%
1737fa
V
5V
3A
OUT