FAN6755W Fairchild Semiconductor, FAN6755W Datasheet - Page 13

• Manufacturer Part Number: FAN6755W
• Description: This highly integrated PWM controller provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN6755W / FAN6755UW • Rev. 1.0.5
V
V
abnormal conditions. Once the V
over-voltage protection voltage (V
t
voltage drops below the UVLO, PWM pulses start again.
Over-voltage conditions are usually caused by open
feedback loops.
Brownout Protection
Since the VIN pin is connected through a resistive
divider to the rectified AC input line voltage, it can also
be used for brownout protection. If V
the PWM output is shut off. When V
0.9V, the PWM output is turned on again. The
hysteresis window for ON/OFF is around 0.2V. The
brownout voltage setting is determined by the potential
divider formed with R
calculate the resistors are shown below:
Thermal Overload Protection
Thermal
dissipation. When the junction temperature exceeds T
+135C, the thermal sensor signals the shutdown logic
and turns off most of the internal circuitry. The thermal
sensor turns internal circuitry on again after the IC’s
junction temperature drops by 25C. Thermal overload
protection is designed to protect the FAN6755W/UW in
the event of a fault condition. For continual operation, do
not exceed the absolute maximum junction temperature
of T
D-VDDOVP
V
DD
DD
IN
J

over-voltage protection prevents damage due to
Over-Voltage Protection
= +150C.
R
, the PWM pulses are disabled. When the V
Lower
overload
R
Lower

R
Upper
protection

Upper
V
AC
and R
2
( ,
unit
DD
DD-OVP
limits
Lower
IN
voltage is over the

is less than 0.7V,
V
IN
. Equations to
), and lasts for
)
reaches over
total
power
(1)
DD
J
=
13
Limited Power Control
The FB voltage is saturated HIGH when the power
supply output voltage drops below its nominal value and
shut regulator (KA431) does not draw current through
the opto-coupler. This occurs when the output feedback
loop is open or output is short circuited. If the FB voltage
is higher than a built-in threshold for longer than t
PWM output is turned off. As PWM output is turned off,
V
delivered from the auxiliary winding.
When V
controller is totally shut down. V
turn-on threshold voltage of 16V through the startup
resistor until PWM output is restarted. This protection
feature continues as long as the over loading condition
persists.
overheating due to overloading conditions.
Noise Immunity
Noise on the current sense or control signal may cause
significant pulse-width jitter, particularly in continuous-
conduction mode. Slope compensation helps alleviate
this problem. Good placement and layout practices
should be followed. Avoiding long PCB traces and
component leads, locating compensation and filter
components near the FAN6755W/UW, and increasing
the power MOS gate resistance improve performance.
DD
begins decreasing since no more energy is
DD
goes below the turn-off threshold (~7.5V), the
This
prevents
the
DD
power
is charged up to the
supply
www.fairchildsemi.com
D-OLP
from
,