FAN6754 Fairchild Semiconductor, FAN6754 Datasheet - Page 12

• Manufacturer Part Number: FAN6754
• Description: The highly integrated FAN6754 PWM controller provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6754 • Rev. 1.0.2
The HV pin can perform current limit to shrink the
tolerance of OCP (Over-Current Protection) under full
range of AC voltage, to linearly current limit curve as
shown in Figure 25. FAN6754 also shrinks the V
level by half to lower the I
heavy-load efficiency.
V
V
abnormal conditions. If the V
voltage protection voltage (V
VDDOVP
voltage drops below the UVLO, then starts again. Over-
voltage conditions are usually caused by open feedback
loops.
DD
DD
0.47
0.46
0.45
0.44
0.43
0.42
0.41
0.39
0.38
over-voltage protection prevents damage due to
Over-Voltage Protection (OVP)
0.4
, the PWM pulses are disabled until the V
Figure 25. Linearly Current Limit Curve
100 120 140 160 180 200 220 240 260 280 300 320 340 360 380
DC Voltage on HV Pin (V)
2
R
DD
SENSE
DD-OVP
voltage is over the over-
loss to increase the
) and lasts for t
limit
DD
D-
12
Thermal Protection
An NTC thermistor, R
be connected from the RT pin to ground. A constant
current I
RT pin can be expressed as V
where I
is smaller, such that V
than 1.035V (V
(t
after 185µs (t
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
controller is totally shut down and, V
discharged to V
average input power. This is called two-level UVLO. V
is cycled again. 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 FAN6754, and increasing the
power MOS gate resistance improve performance.
D-OLP
D-OTP1
, PWM output is turned off. As PWM output is
). If V
RT
RT
DD
is 100µA. At high ambient temperature, R
is output from the RT pin. The voltage on the
goes below the turn-off threshold (9V) the
RT
DD
D-OTP2
is less than 0.7V (V
begins decreasing.
RTTH1
DD-OLP
).
), the PWM turns off after 16ms
NTC
RT
(6.5V) by I
, in series with resistor R
decreases. When V
RT
=I
RTTH2
DD-OLP
RT
DD
• (R
), PWM turns off
is continuously
to lower the
NTC
www.fairchildsemi.com
RT
+ R
is less
A
, can
PTC
NTC
DD
),