FAN6749 Fairchild Semiconductor, FAN6749 Datasheet - Page 13

• Manufacturer Part Number: FAN6749
• Description: The FAN6749 highly integrated PWM controller enhances the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2012 Fairchild Semiconductor Corporation
FAN6749 • Rev. 1.0.0
Short Circuit Protection (SCP)
The V
power supply is shorted because the impedance at
secondary windings becomes far lower than at auxiliary
winding and currents choose the low-impedance path.
When V
t
latched off.
Under-Voltage Lockout (UVLO)
As shown in Figure 33, as long as protection is not
triggered, the turn-off threshold of V
at V
level to terminate PWM gate switching is changed to
V
below V
operating current from V
down the discharge of V
This delays restart after shutdown by protection to
minimize the input power and voltage/current stress of
switching devices during fault condition.
D-SCP
DD-OFF
Figure 31. Over-Current Protection Circuit
UVLO
; if V
0.55
0.45
0.35
DD
0.5
0.4
0.3
FB
(11V), as shown in Figure 34. When V
DD-OFF
70
(6.5V). When a protection is triggered, the V
voltage decreases every time the output of the
is larger than V
Figure 32. V
DD
, the switching is terminated and the
is lower than V
110
OCP
150
DD
FB-OLP
DD
is reduced to I
vs. Line Voltage
until V
over a debounce time of
DD-SCP
190
DD
DD
, PWM output is
is fixed internally
reaches V
230
DD-OLP
DD
, to slow
270
DD-OLP
drops
DD
.
13
Leading-Edge Blanking (LEB)
Each time the power MOSFET is switched on, a turn-on
spike occurs on the sense resistor. To avoid premature
termination of the switching pulse, a leading-edge
blanking time, t
period, the current-limit comparator is disabled and
cannot switch off the gate driver.
Gate Output / Soft Driving
The BiCMOS output stage has a fast totem-pole gate
driver. The output driver is clamped by an internal 14.5V
Zener diode to protect power MOSFET gate from over
voltage. A soft driving is implemented to minimize
electromagnetic interference (EMI) by reducing the
switching noise.
V
V
over-voltage exceeding the IC voltage rating. When the
V
This protection is typically caused by an open circuit in
the secondary-side feedback network.
V
V
V
DD
DD
DD
DD-OFF
DD-ON
DD-OLP
over-voltage protection prevents IC damage from
voltage exceeds 29V, the protection is triggered.
Over-voltage Protection (OVP)
Figure 34. V
Figure 33. V
LEB
, is introduced. During this blanking
DD
DD
UVLO in Protection Mode
UVLO in Normal Mode
7.5V
17V
11V
www.fairchildsemi.com
t