FAN6753_11 Datasheet, PDF (10/12 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Functional Description (Continued)
Limited Power Control
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 t_D-OLP, PWM output is turned off. As
PWM output is turned off, V_DD begins decreasing.
When V_DD goes below the turn-off threshold (~9.5V), the
controller is totally shut down. V_DD 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, V_DD gradually
drops to the UVLO voltage (around 7.5V). Then V_DD
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.
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 FAN6753, and increasing the
power MOS gate resistance also improve performance.
© 2009 Fairchild Semiconductor Corporation
FAN6753 • Rev. 1.0.3
10
www.fairchildsemi.com