LT1737IGN(RevA) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT1737IGN Datasheet PDF : 28 Pages
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LT1737
TYPICAL APPLICATIO S
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 VBE of Q1 is converted to a current by R2 and then back
to a voltage at R3. Or, stated mathematically:
( ) VFB =
VFLBK
–
VBE
⎛ R3⎞
⎝⎜ R2⎠⎟
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.
5VIN APPLICATION
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
2VBEs. Thus when operating at a worst case 4.5V supply,
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
Q1
2N3906
R2
11.0k
1%
D2
BAS16
VIN
C1A +
220µF
10V
C1B +
220µF
10V
T1
COILTRONICS
VP5-0083
D1
MBRD835L
D4
1N5240
C8
1µF
25V
D3
Z5U MBR0520
R8
10Ω 12 11 10
5%
C6 1 • 2 • 3 •
4.7nF
50V
X7R
•4 •5 •6
987
R9 + C2A
15Ω
220µF
5%
10V
C7
2.2nF
50V
X7R
C2B +
220µF
10V
R3
3.01k
1%
9
10
15
8
FB
7
VC
3VOUT UVLO
VCC
LT1737
16
GATE
ISENSE 2
C3 OSCAP SFST tON ENDLY MINENAB ROCMP RCMPC SGND PGND
1nF
6
3 14 13
12
4
5
11 1
25V
X7R
C5
47pF
50V
NPO
R4 R5
R6
R7
75k 51k 51k 2.2k
5% 5% 5% 5%
C4
0.1µF
25V
Z5U
C9
1nF
25V
X7R
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
M1
SI9804
M2
SI9804
R11
51Ω
5%
R1
0.02Ω
1737 F15
VOUT
5V
R10 3A
270Ω
5%
Figure 15. 4.5V-5.5V to Isolated 5V Converter
1737fa
24
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