Datasheet

LTC3900
12
3900fb
applicaTions inForMaTion
the driver dissipation, the total gate charge Q
G
is used.
This parameter is found on the MOSFET manufacturers
data sheet.
The power dissipated in each LTC3900 MOSFET driver
is:
P
DRIVER
= Q
G
• V
CC
• f
SW
where f
SW
is the switching frequency of the converter.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3900 for your layout:
1. Connect the 4.7µF bypass capacitor as close as possible
to the V
CC
and GND pins.
2. Connect the two MOSFET drain terminals directly to
the transformer. The two MOSFET sources should be as
close together as possible.
3. Keep the timer, SYNC and V
CC
regulator circuit away
from the high current path of Q3, Q4 and T1.
4. Place the timer capacitor, C
TMR
, as close as possible
to the LTC3900.
5. Keep the PCB trace from the resistors R
CS1
, R
CS2
and
R
CS3
to the LTC3900 CS
+
/CS
–
pins as short as possible.
Connect the other ends of the resistors directly to the drain
and source of the MOSFET, Q4.
•
••
••
LTC3900
8V
BIAS
V
B1
V
B1
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
1µF
15k
560R
C
OUT
100µF
3x
0.1µF
2.2µF
1nF
12V
Q2
PH3230
2x
Q3
PH3230
2x
18V
NEC
PS2701
Q4470-B
V
B1
C16
10pF
C13
1µF
C14
33nF
C15
6.8nF
82k
47k
3900 TA01
L1
SD_V
SEC
OUT
LT1952
7 14
R
OSC
V
IN
3 15
BLANK GND
9 8
SS_MAXDC PGND
5 13
DELAY
12
V
R
= 2.5V OC
6 11
COMP I
SENSE
1 10
FB = 1.23V SOUT
2 16
Si7846
1
GND
6
CS
+
2
V
CC
4
CS
–
7
SYNC
8
TIMER
370k
0.010R
1nF
470Ω
39k
13.2k
115k
27k
0.22µF
0.1µF
10k59k
33k
2.2k
22k
L1: PA0713, PULSE ENGINEERING
ALL CAPACITORS X7R, CERAMIC, TDK
R24
27.4k
1%
R25
6.04k
1%
R23
3.3k
6
5
4
1
2
3
LT4430
V
IN
GND
OC
OPTO
COMP
FB
8V BIAS
R22
270Ω
Typical applicaTions
36V to 72V, 3.3V at 40A Synchronous Forward Converter