ltc3900 Linear Technology Corporation, ltc3900 Datasheet - Page 10

• Manufacturer Part Number: ltc3900
• Description: Synchronous Rectifier Driver For Forward Converters
• Manufacturer: Linear Technology Corporation

LTC3900
APPLICATIO S I FOR ATIO
tion delay, make the positive and negative magnitude of
the square wave much greater than the ±1.4V SYNC
threshold.
V
The V
rectifying the transformer secondary winding as shown in
Figure 9. The Zener diode D
(V
ohms), in series with the base of Q
surpress high frequency oscillations depending on Q
selection.
The LTC3900 has an UVLO detector that pulls the drivers
output low if V
hysteresis to prevent chattering.
In a typical forward converter, the secondary-side circuits
have no power until the primary-side controller starts
operating. Since the power for biasing the LTC3900 is
derived from the power transformer T1, the LTC3900 will
initially remain off. During that period (V
output rectifier MOSFETs Q3 and Q4 will remain off and the
MOSFETs body diodes will conduct. The MOSFETs may
experience very high power dissipation due to a high
voltage drop in the body diodes. To prevent MOSFET
damage, V
10
CC
Z
CONTROLLER
– 0.7V). A resistor, R
Regulator
CC
PRIMARY
supply for the LTC3900 can be generated by peak
SECONDARY
SG
SG
CC
WINDING
voltage greater than 4.1V should be provided
Figure 8. Symmetrical SYNC Drive
CC
T1
74HC14
< 4.1V. The UVLO detector has 0.5V of
Figure 9. V
MBR0540
U
0.1µF
D3
R1
470Ω
C1
220pF
74HC14
B
U
(on the order of a few hundred
CC
Z
74HC132
Regulator
sets the output voltage to
R
2k
Z
SYNC
REG
D
7.5V
10Ω
Z
W
R
B
may be required to
T2
CC
Q
BCX55
3900 F09
REG
C
4.7µF
< 4.1V), the
VCC
R
470Ω
V
SYNC
CC
LTC3900
SYNC
U
3900 F08
REG
’s
quickly. The V
provide power for the LTC3900 within the first few switch-
ing pulses of the primary controller, preventing overheat-
ing of the MOSFETs.
MOSFET Selection
The required MOSFET R
based on allowable power dissipation and maximum re-
quired output current.
The body diodes conduct during the power-up phase,
when the LTC3900 V
FG signals stay low and the inductor current flows through
the body diodes. The body diodes must be able to handle
the load current during start-up until V
The LTC3900 drivers dissipate power when switching
MOSFETs. The power dissipation increases with switch-
ing frequency, V
the driver dissipation, the total gate charge Q
This parameter is found on the MOSFET manufacturers
data sheet.
The power dissipated in each LTC3900 MOSFET driver is:
where f
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
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
from the high current path of Q3, Q4 and T1.
4. Place the timer capacitor, C
the LTC3900.
5. Keep the PCB trace from the resistors R
R
Connect the other ends of the resistors directly to the drain
and source of the MOSFET, Q4.
CS3
P
DRIVER
to the LTC3900 CS
SW
CC
and GND pins.
is the switching frequency of the converter.
= Q
G
CC
CC
• V
supply circuit shown in Figure 9 will
and size of the MOSFETs. To calculate
CC
CC
• f
supply is ramping up. The CG and
SW
+
/CS
DS(ON)
–
TMR
pins as short as possible.
CC
, as close as possible to
should be determined
regulator circuit away
CC
reaches 4.1V.
CS1
, R
G
is used.
CS2
and
3900f