FAN6755WMY Datasheet, PDF (13/17 Pages) Fairchild Semiconductor – mWSaver™ PWM Controller

V_DD 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, the internal startup circuit
turns on, and VDD is charged to VDD-ON to restart IC.
Feedback Impedance Switching
FAN6755W/UW actively varies FB-pin impedance
(ZFB) to reduce no-load power consumption. This
technique can further reduce operating current of the
controller when FB-pin voltage drops below VFB-ZDC.
Figure 22 exhibits the range that ZFB changes. When
VFB is lower than VFB-ZDC, PWM switching is stopped
and ZFB is switched from 15 kΩ to 90 kΩ. On the other
hand, ZFB is switched from 90 kΩ to 15 kΩ when VFB is
higher than VFB-ZDCR.
f_osc (kHz)
Proprietary
Z_FB
=90k
Z_FB
=15k
VFB-ZDC VFB-ZDCR
V_FB(V)
Figure 22. ZFB-Switching Activating Range
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
0.7 V, the PWM output is shut off. When VIN reaches
over 0.9 V, the PWM output is turned on again. The
hysteresis window for ON/OFF is around 0.2 V. The
brownout voltage setting is determined by the potential
divider formed with RUpper and RLower. Equations to
calculate the resistors are shown below:
V_IN

R_Lower
R_Lower  R_Upper
 V_AC
2, (unit  V)
(1)
Thermal Overload Protection
Thermal overload protection limits total power
dissipation. When the junction temperature exceeds TJ=
+140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,
the controller should not exceed the absolute maximum
junction temperature of TJ = +140C.
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.
As the protection is triggered, VDD enters into UVLO
mode. 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 gate resistor from GATE pin to MOSFET improve
performance.
© 2009 Fairchild Semiconductor Corporation
FAN6755W / FAN6755UW • Rev. 1.0.7
13
www.fairchildsemi.com