LTC3900_1 Datasheet, PDF (12/20 Pages) Linear Technology – Synchronous Rectifier Driver for Forward Converters

LTC3900
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.
typical applications
36V to 72V, 3.3V at 40A Synchronous Forward Converter
0.22µF
0.1µF
+V_IN
36V TO 72V
V_B1
47k
82k
18V
BCX55
12V
PA0912.002
••
BAS516
0.1µF
Q2
PH3230
2x
Q3
PH3230
2x
L1
V_OUT
3.3V, 40A
C_OUT
100µF
10k
3x
370k
13.2k
115k
27k
7
SD_V_SEC
3
R_OSC
9 _BLANK
5
SS_MAXDC
59k 10k
LT1952
6
33k
V_R = 2.5V
1
COMP
2
FB = 1.23V
22k
2.2k
14
OUT
15
V_IN
GND ⁸
13
PGND
DELAY ¹²
11
OC
10
I_SENSE
16
SOUT
2.2µF
•
V_B1
Si7846
BAT760
1nF
1µF
8V
BIAS
LTC3900
5 _FG
CG ³
6 _GND
CS^{+ 1}
4 _VCC
CS^{– 2}
8 _{SYNC TIMER} 7
15k
10k
10k
1nF
39k
0.010R
1nF
8V
BIAS
470Ω
220pF
560R
Q4470-B
R22
270Ω
V_B1
NEC
C16
PS2701
10pF
L1: PA0713, PULSE ENGINEERING
ALL CAPACITORS X7R, CERAMIC, TDK
8V BIAS
C13
1µF
C14
33nF
1
6
V_IN OPTO
2 ^LT4430 5
GND COMP
3 _OC
FB ⁴
R23
3.3k
C15
6.8nF
R24
27.4k
1%
3900 TA01
R25
6.04k
1%
3900fb
12