FAN6747 Fairchild Semiconductor, FAN6747 Datasheet - Page 13

• Manufacturer Part Number: FAN6747
• Description: The highly integrated FAN6747 PWM controller provides several features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6747 • Rev. 1.0.3
Built-In Slope Compensation
The sensed voltage across the current-sense resistor is
used for peak-current-mode control and pulse-by-pulse
current limiting. Built-in slope compensation improves
stability
FAN6747 inserts a synchronized, positive-going, ramp
at every switching cycle.
Constant Output Power Limit
When the SENSE voltage across sense resistor R
reaches the threshold voltage, the output GATE drive is
turned off after a small delay, t
an additional current proportional to t
the delay is nearly constant regardless of the input
voltage V
additional current and the output power limit is higher
than under low input line voltage. To compensate this
variation for a wide AC input range, a power-limiter is
controlled by HV pin to solve the unequal power-limit
problem. The power limiter is fed to the inverting input of
the OCP comparator. This results in a lower current limit
at high-line input than at low-line input.
IN
and
, higher input voltage results in a larger
prevents
sub-harmonic
PD
. This delay introduces
PD
• V
IN
/ L
oscillation.
P
. Since
S
13
Over-Temperature Protection (OTP)
A NTC thermistor, R
connected from the RT pin to GND pin. A constant
current I
pin can be expressed as V
I
5V by internal circuitry. As high ambient temperatures
occur, R
When V
PWM turns off after t
V
after a short time. If the RT pin is not connected to the
NTC resistor for over temperature protection, it is
recommended to connect one 100KΩ resistor to ground
to prevent noise interference. This pin is limited by
internal clamping circuit.
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 FAN6747, and increasing the
power MOS gate resistance improve performance.
RT
DD
is 100µA, the headroom of V
under 0.7V, is used for fast shut down of FAN6747
RT
RT
NTC
is output from this pin. The voltage of the RT
is less than 1.05V(V
is smaller, such that the V
NTC
D_OTP-LATCH
, in series with a resistor, R
RT
= I
RT
OTP
. The other threshold,
RT
• (R
) but over 0.7V, the
is limited at around
NTC
RT
+ R
www.fairchildsemi.com
decreases.
A
), where
A
, is