LTC3900
11
3900fb
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
CC
Regulator
The V
CC
supply for the LTC3900 can be generated by peak
rectifying the transformer secondary winding as shown
in Figure 9. The Zener diode D
Z
sets the output voltage to
(V
Z
– 0.7V). A resistor, R
B
(on the order of a few hundred
ohms), in series with the base of Q
REG
may be required
to surpress high frequency oscillations depending on
Q
REG
’s selection.
The LTC3900 has an UVLO detector that pulls the drivers
output low if V
CC
< 4.1V. The UVLO detector has 0.5V of
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
CC
< 4.1V), the
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
CC
voltage greater than 4.1V should be provided
quickly. The V
CC
supply circuit shown in Figure 9 will pro-
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
DS(ON)
should be determined based
on allowable power dissipation and maximum required
output current.
The body diodes conduct during the power-up phase, when
the LTC3900 V
CC
supply is ramping up. The CG and 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
CC
reaches 4.1V.
The LTC3900 drivers dissipate power when switching
MOSFETs. The power dissipation increases with switch-
ing frequency, V
CC
and size of the MOSFETs. To calculate
Figure 9. V
CC
Regulator
Figure 8. Symmetrical SYNC Drive
R
SYNC
470Ω
T2
LTC3900
SYNC
3900 F08
74HC14
74HC14
74HC132
R1
470Ω
C1
220pF
SYNC
SG
PRIMARY
CONTROLLER
SG
3900 F09
D3
MBR0540
T1
SECONDARY
WINDING
0.1µF
R
Z
2k
R
B
10Ω
Q
REG
BCX55
C
VCC
4.7µF
V
CC
D
Z
7.5V
