LTC3721-2 LINER [Linear Technology], LTC3721-2 Datasheet - Page 11

• Manufacturer Part Number: LTC3721-2
• Description: Synchronous Rectifier Driver for Forward Converters
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
An alternative method of generating the SYNC pulse is
shown in Figure 8. This circuit produces square SYNC
pulses with amplitude dependent on the logic supply
voltage. The SYNC pulse width can be adjusted with R1
and C1 without affecting the pulse amplitude.
For nonisolated applications, the SYNC input can be driven
directly by a bipolar square pulse. To reduce the propa-
gation 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
to surpress high frequency oscillations depending on
Q
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
CC
REG
Z
– 0.7V). A resistor, R
Regulator
CONTROLLER
’s selection.
CC
PRIMARY
supply for the LTC3900 can be generated by peak
SG
SG
Figure 8. Symmetrical SYNC Drive
CC
74HC14
< 4.1V. The UVLO detector has 0.5V of
R1
470
C1
220pF
B
(on the order of a few hundred
74HC14
Z
74HC132
sets the output voltage to
SYNC
REG
may be required
T2
R
470
SYNC
LTC3900
SYNC
3900 F08
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 dam-
age, V
quickly. The V
vide power for the LTC3900 within the first few switching
pulses of the primary controller, preventing overheating
of the MOSFETs.
MOSFET Selection
The required MOSFET R
on allowable power dissipation and maximum required
output current.
The body diodes conduct during the power-up phase, when
the LTC3900 V
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
CC
SECONDARY
voltage greater than 4.1V should be provided
WINDING
CC
CC
T1
CC
supply circuit shown in Figure 9 will pro-
supply is ramping up. The CG and FG
Figure 9. V
MBR0540
and size of the MOSFETs. To calculate
D3
0.1µF
DS(ON)
CC
should be determined based
Regulator
R
2k
Z
D
7.5V
10
Z
R
B
CC
LTC3900
CC
reaches 4.1V.
Q
BCX55
3900 F09
REG
C
4.7µF
< 4.1V), the
VCC
V
CC
11
3900fb