FAN6751MR Fairchild Semiconductor, FAN6751MR Datasheet - Page 11

• Manufacturer Part Number: FAN6751MR
• Description: The highly integrated FAN6751 series of PWM controllers provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN6751MR • Rev. 1.0.0
V
V
damage due to abnormal conditions. Once the V
voltage is over the over-voltage protection voltage (V
OVP
disabled until the V
then starts 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 the V
less than 0.7V, the PWM output is shut off. If the V
voltage over 0.92V, the PWM output is turned on again.
The hysteresis window for on/off is around 0.22V. The
brownout voltage setting is determined by the potential
divider formed with RUpper and RLower. To calculate
the resistors:
Thermal-Overload Protection
Thermal-overload
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 FAN6751
in the event of a fault condition. For continual operation,
do not exceed the absolute maximum junction
temperature rating of T
V
DD
DD
IN
), and lasts for t
Over-Voltage Protection (OVP)
=
over-voltage protection is built-in to prevent
R
Lower
R
Lower
+
R
Upper
DD
protection
D-VDDOVP
J
voltage drops below the UVLO,
= +150°C.
×
V
AC
, the PWM pulses are
( ,
unit
limits
=
V
)
total
IN
voltage is
(1)
power
DD-
DD
IN
J
11
Limited Power Control
The FB voltage increases every time the output of the
power supply is shorted or overloaded. If the FB voltage
remains higher than a built-in threshold for longer than
t
turned off, V
When V
the controller is totally shut down. V
the turn-on threshold voltage of 16.5V through the
startup resistor until PWM output is restarted. This
protection feature continues as long as the overloading
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 FAN6751, and increasing the
power MOS gate resistance improve performance.
D-OLP
, PWM output is turned off. As PWM output is
DD
goes below the turn-off threshold (~10.5V),
DD
begins decreasing.
DD
is charged up to
www.fairchildsemi.com