Datasheet
18
LT3781
3781f
Figure 9. 36V to 72V DC in to 3.3V/20A Isolated Synchronous Forward Converter with Fast Start and Differential Sense
V
CC
13
2
1
5
1µF 82pF
OVLO
SHDN
1.24k
1%
73.2k
1%
20k
10k
267k
0.25W
C26
68µF
20V
5V
REF
6
F
SET
4700pF
8
SS
10
14
BAS21
BAT54
T2
S
S
BAS21
BAT54
BAT54
Q12
ZVN3310F
9
V
C
PGND
V
FB
374
THERM
LT3781
SYNC SGND
52.3k
1%
10Ω
1k
3k
1k
1k
100Ω
9V
10k
2k
0.25W
Q13
FZT690B
4.7µF
16V
0.22µF
50V
S
MMBZ5240BLT1
10V
1
3.3Ω
10k
5V
REF
ISO1
MOC207
7
143
3
1
4
6
5
14
15
6
5
82
3300pF
4700pF
0.1µF
5V
REF
12
SG
+
0.1µF
1nF
11
SENSE
15
BG
18
BSTREF
19
TG
20
BAS21
0.1µF 100V
L3 1mH
BLKSENS
V
BST
220pF
C4
1.5µF
100V
C3
1.5µF
100V
L1
3.3µH
0.022µF
1000pF
••
SYNC V
FB
OVPIN
MARGIN
I
COMP
V
DD
OPTODRV
V
AUX
0.1µF
12
I
SNS
11
I
SNSGND
16
FG
2
CG
PGND GND
LTC1698
PWRGD
6
8
9
7
13
1.24k
1%
1.78k
1%
3.01k
1%
SENSE
+
SENSE
–
TRIM
100Ω
0.25W
5
6
4
2
3
1
7
9V
V
OUT
+
2.43k
1%
1043
V
COMP
1698 F12
3.01k
1%
3.01k
1%
3.01k
1%
3.01k
1%
100Ω
0.25W
1k
0.33µF
MBR0530
0.025Ω
1/2W
3
4
Q3
1000pF
100V
C1
2200pF
250V
1000pF
100V
Q5, Q14
FDS6680A
×2
Q6, Q15, Q17
FDS6680A
×3
MURS120T3
MURS120T3
10Ω
2
7
V
SEC
5
•
••
1
Q1
T1
10Ω
MMBT3906LT1
MMBT3906LT1
V
IN
+
V
IN
–
C2
1.5µF
100V
V
OUT
+
V
OUT
–
S
C5 TO C8
330µF
10V
×4
10Ω
0.25W
10Ω
0.25W
4.7Ω
L2
2.35µH
330pF
+
62k
0.25W
MMBZ5248LT1
18V
MMBT3904LT1
47k
4.7µF
5V
REF
MMBD914LT1
Q7
NDT410EL
1.5k
0.25W
1.5k
0.25W
–
+
LT1006S8
C1: MURATA ERIE GHM3045X7R222K-GC
C2, C3, C4: VITRAMON VJ1825Y155MXB
C5 TO C8: 330µF 10V KEMET T510X337K010AS
OR 330µF 6.3V KEMET T520D337M006AS
C26: AVX TPSE686M020R0150
ISO1: FAIRCHILD MOC207
L1: COILCRAFT DO1608C-332
L2: PULSE P1977 PLANAR INDUCTOR
L3: COILCRAFT DO1608C-105
Q1, Q3: SILICONIX Si4486EY
Q5, Q6, Q14,Q15,Q17: FAIRCHILD FDS6680A
Q7: FAIRCHILD NDT410EL
Q12: ZETEX ZVN3310F
Q13: ZETEX FZT690
T1: PULSE P1976 PLANAR TRANSFORMER
(FUNCTIONAL INSULATION) OR
PULSE PA-0191 (BASIC INSULATION)
T2: MIDCOM 31264R (FUNCTIONAL INSULATION) OR
MIDCOM 31323R (BASIC INSULATION)
TYPICAL APPLICATIO S
U