LT1952
22
1952f
APPLICATIO S I FOR ATIO
WUU
U
Figure 18. 36V to 72V, 3.3V at 40A Synchronous Forward Converter
Figure 17. LT1952-Based Synchronous Forward Converter
Efficiency vs Load Current (For Circuit in Figure 18)
36V to 72V Input, 3.3V 40A Converter
An LT1952-based synchronous forward converter pro-
vides the ideal solution for power supplies requiring high
efficiency at low output voltages and high load currents.
The 3.3V 40A solution in Figure 18 achieves peak efficien-
cies of 92.5% (Figure 17) by minimizing power loss due to
rectification at the output. Synchronous rectifier control
output SOUT, with programmable delay, optimizes timing
control for a secondary side synchronous MOSFET con-
troller (LTC3900) which results in high efficiency synchro-
nous rectification. The LT1952 uses a precision current
limit threshold at the OC pin combined with a soft-start
hiccup mode to provide low stress output short circuit
protection. The maximum output current will vary only
10% over the full V
IN
range. During short circuit the
average power dissipation of the circuit will be lower than
15% of maximum rated power thanks to a soft-start
OUTPUT CURRENT (A)
010
EFFICIENCY (%)
50
1952 F17
20
30
40
94
93
92
91
90
89
88
87
86
V
IN
= 48V
V
OUT
= 3.3V
f
OSC
= 300kHz
•
••
••
LTC3900
8V
BIAS
V
U1
V
U1
220pF
BAT760
8V
BIAS
3
FG
5
CG
+V
IN
36V TO 72V
PA0912.002
BAS516
BCX55
V
OUT
3.3V, 40A
10k
10k
10k
1nF
8V
BIAS
1µF
15k
560Ω
C
OUT
100µF
3x
0.1µF
2.2µF
1nF
12V
Q2
PH3230
2x
Q3
PH3230
2x
+
–
18V
270Ω
PS8101
T2
Q4470-B
2.2nF
V
U1
18k
249k 80.6k
1µF
0.1µF
10k
10k
82k
47k
4
3
5
4
8
2
1
LT1009
LT1797
1952 F18
L1
SD_V
SEC
OUT
LT1952
714
R
OSC
V
IN
315
BLANK GND
98
SS_MAXDC PGND
513
DELAY
12
V
R
= 2.5V OC
611
COMP I
SENSE
110
FB = 1.23V SOUT
216
•
Si7846
1
GND
6
CS
+
2
V
CC
4
CS
–
7
SYNC
8
TIMER
370k
10mΩ
1µF
470Ω
39k
13.2k
115k
27k
0.22µF
0.1µF
10k59k
33k
2.2k
22k
L1: PA0713, PULSE ENGINEERING
ALL CAPACITORS X7R, CERAMIC, TDK
T2: COILCRAFT
controlled hiccup mode. This allows a significant reduc-
tion in power component sizing using the LT1952-based
converter.
