AN-6754 Datasheet, PDF (1/8 Pages) Fairchild Semiconductor – Design Guideline to Replace SG6742

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AN-6754
Design Guideline to Replace SG6742 with FAN6754
1. Introduction
FAN6754 is a highly integrated PWM controller featuring
green mode, frequency hopping, constant power limit, and a
number of protection functions. The green-mode and burst-
mode functions, with a low operating current, maximize the
light-load efficiency so that the power supply can meet
stringent standby power regulations. The frequency hopping
reduces the Electro-Magnetic Emission (EMI) by spreading
the frequency spectrum. The constant power limit function
minimizes the component stress in abnormal condition and
helps to optimize the power stage. Protection functions; such
as brownout protection, overload / open-loop protection
(OLP), over-voltage protection (OVP), and over-
temperature protection (OTP); are fully integrated into
FAN6754, which improves the reliability of a switched
mode power supply without increasing system cost. This
application note explains how to replace old-generation
PWM controller SG6742 with the newer FAN6754. These
two devices have the same pin configuration and
replacement can be achieved without PCB layout changes.
However, because functional improvements have been made
to FAN6754 for higher efficiency and better performance,
several external components should be changed accordingly.
Table 1 summarizes the difference between these two
devices. The pulse-by-pulse current limit threshold voltage
is reduced almost by half to reduce the current sensing loss,
which results in 0.4~0.5% efficiency improvement. The
operating current is also reduced to achieve lower standby
power consumption, which allows less than 100mW standby
power consumption for most of the design. The typical
application circuit and internal block diagram are shown in
Figure 1 and Figure 2, respectively.
Table 1. Comparison table for SG6742 and FAN6754
SG6742
Brownout Protection
No
Line Voltage compensation for Pulse-by-
Pulse Current Limit (V_limit-L / V_limit-H)
Sense Pin Short-Circuit Protection
Saw-Limit
(0.9V/0.56V)
VSENSE<0.15V Longer than 150µs
Gate Drive Clamping Voltage
18V
FB Impedance (Z_FB)
Operating Current (I_DD-OP)
Leading Edge Blanking Time (t_LEB)
Soft Start
5KΩ
2.7mA
150ns
5ms
Maximum Duty Cycle
65%
V_FB-G / V_FB-N for Green Mode
V_ZDC / V_ZDCR for Burst Mode
2.4V / 3.0V
1.6V / 1.7V
FAN6754
Line Sensing Using HV Pin
Adjusted by HV Pin
(0.46V/0.39V)
No
13V
15.5KΩ
1.7mA
280ns
8ms
90%
2.3V / 2.8V
2.0V / 2.1V
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 11/18/09
www.fairchildsemi.com