FAN6749 Fairchild Semiconductor, FAN6749 Datasheet - Page 14

• Manufacturer Part Number: FAN6749
• Description: The FAN6749 highly integrated PWM controller enhances the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2012 Fairchild Semiconductor Corporation
FAN6749 • Rev. 1.0.0
Soft-Start
FAN6749
progressively increases the cycle-by-cycle current limit
level of the MOSFET for 6ms during startup to establish
the correct working conditions for transformers and
capacitors to operate.
Over-Temperature Protection (OTP)
The RT pin provides adjustable Over-Temperature
Protection (OTP) and an external latch-triggering
function. For OTP application, an NTC thermistor, R
usually in series with a resistor, R
between the RT pin and ground. The internal current
source, I
At high ambient temperature, R
reduces V
longer than t
and FAN6749 enters Latch Mode protection.
V
RT

I
RT
RT

(
RT
, (100µA) introduces voltage on RT as:
R
has
NTC
. When V
D-OTP1

R
an
A
)
(14ms), the protection is triggered
RT
internal
is lower than V
soft-start
NTC
decreases, which
RTTH1
A
, is connected
circuit
(1.050V) for
that
NTC
(4)
,
14
The OTP can be also trigged by pulling down the RT pin
voltage using an opto-coupler or transistor. Once V
less than V
the protection is triggered and FAN6749 enters Latch
Mode protection.
When V
disabled for lower power consumption. If OTP is not
used, place a 100kΩ resistor between this pin and
ground to prevent noise interference.
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 FAN6749, and increasing the
power MOS gate resistance improve performance.
FB
RTTH2
falls below 1.9V, the RT pin function is
(0.7V) for longer than t
D-OTP2
www.fairchildsemi.com
(165µs),
RT
is