
2009 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FAN6755W / FAN6755UW Rev. 1.0.5
13
FAN6755W
/
FAN6755UW
—
mWSaver
Controller
VDD Over-Voltage Protection
VDD over-voltage protection prevents damage due to
abnormal conditions. Once the VDD voltage is over the
over-voltage protection voltage (VDD-OVP), and lasts for
tD-VDDOVP, the PWM pulses are disabled. When the VDD
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 VIN is less than 0.7V,
the PWM output is shut off. When VIN reaches over
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 RUpper and RLower. Equations to
calculate the resistors are shown below:
)
V
unit
(
,
V
R
V
AC
Upper
Lower
IN
2
(1)
Thermal Overload Protection
Thermal
overload
protection
limits
total
power
dissipation. When the junction temperature exceeds TJ=
+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 TJ = +150C.
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 tD-OLP,
PWM output is turned off. As PWM output is turned off,
VDD begins decreasing since no more energy is
delivered from the auxiliary winding.
When VDD goes below the turn-off threshold (~7.5V), the
controller is totally shut down. VDD is charged up to the
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. This prevents the power supply from
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.