Functional Description (Continued)
Limited Power Control
Noise Immunity
The feedback (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.
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 FAN6753, and increasing the
power MOS gate resistance also improve performance.
When VDD goes below the turn-off threshold (~9.5V),
the controller is totally shut down. VDD is charged up to
the turn-on threshold voltage of 15.5V through the
startup resistor until PWM output is restarted. This
protection feature continues as long as the overloading
condition persists.
Over-Temperature Protection
(Internal OTP)
The built-in temperature-sensing circuit shuts down
PWM output once the junction temperature exceeds
135°C. While PWM output is shut down, VDD gradually
drops to the UVLO voltage (around 7.5V). Then VDD
charges up to the startup threshold voltage of 15.5V
through the startup resistor until PWM output is
restarted. This “hiccup” mode protection occurs
repeatedly as long as the temperature remains above
130°C. The temperature hysteresis window for the OTP
circuit is 25°C.
© 2009 Fairchild Semiconductor Corporation
FAN6753 • Rev. 1.0.1
www.fairchildsemi.com
10
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
