FAN6751MR Datasheet, PDF (11/13 Pages) Fairchild Semiconductor – Highly-Integrated Green-Mode PWM Controller

V_DD Over-Voltage Protection (OVP)
VDD over-voltage protection is built-in to prevent
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 until the VDD voltage drops below the UVLO,
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 VIN voltage is
less than 0.7V, the PWM output is shut off. If the VIN
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:
V_IN
=
R_Lower
R_Lower + R_Upper
×V_AC , (unit
=V)
(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 FAN6751
in the event of a fault condition. For continual operation,
do not exceed the absolute maximum junction
temperature rating of TJ = +150°C.
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
tD-OLP, PWM output is turned off. As PWM output is
turned off, VDD begins decreasing.
When VDD goes below the turn-off threshold (~10.5V),
the controller is totally shut down. VDD is charged up to
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.
© 2008 Fairchild Semiconductor Corporation
FAN6751MR • Rev. 1.0.0
11
www.fairchildsemi.com