FAN6754B Fairchild Semiconductor, FAN6754B Datasheet - Page 12

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FAN6754B

Manufacturer Part Number
FAN6754B
Description
The highly integrated FAN6754A PWM controller provides several features to enhance the performance of flyback converters
Manufacturer
Fairchild Semiconductor
Datasheet
© 2011 Fairchild Semiconductor Corporation
FAN6754B • Rev.1.0.0
The HV pin can perform current limit to shrink the
tolerance of Over-Current Protection (OCP) under full
range of AC voltage, to linearly current limit curve, as
shown in 0. FAN6754B also shrinks the V
half to lower the I
load efficiency.
V
V
abnormal conditions. If the V
voltage protection voltage (V
VDDOVP
below the UVLO, then starts again. Over-voltage
conditions are usually caused by open feedback loops.
Thermal Protection
An NTC thermistor, R
be connected from the RT pin to ground. A constant
current, I
the RT pin can be expressed as V
where I
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 V
Figure 25. Linearly Current Limit Curve
100 120 140 160 180 200 220 240 260 280 300 320 340 360 380
RT
RT
is 100µA. At high ambient temperature, R
, is output from the RT pin. The voltage on
2
R
SENSE
NTC
DC Voltage on HV Pin (V)
, in series with resistor R
loss to increase the heavy-
DD
DD-OVP
voltage is over the over-
RT
=I
) and lasts for t
RT
• (R
limit
NTC
DD
level by
+ R
A
drops
, can
PTC
NTC
D-
),
12
is smaller, such that V
than 1.035V (V
(t
off after 185µs (t
NTC
connecting a series one 100K剸 resistor to ground to
prevent from noise interference is recommended. This
pin is limited by an internal clamping circuit.
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 FAN6754B, and increasing the
power MOS gate resistance improve performance.
D-OLP
D-OTP1
, PWM output is turned off. As PWM output is
). If V
resistor
DD
goes below the turn-off threshold (10V) the
RT
DD
is less than 0.7V (V
begins decreasing.
RTTH1
D-OTP2
DD-OLP
for
), the PWM turns off after 16ms
). If the RT pin is not connected to
RT
(6.5V) by I
over-temperature
decreases. When V
RTTH2
DD-OLP
DD
), the PWM turns
is continuously
to lower the
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protection,
RT
is less
DD