AN-6754A Datasheet, PDF (1/9 Pages) Fairchild Semiconductor – Design Guideline to Replace SG6742 with FAN6754A

www.fairchildsemi.com
AN-6754A
Design Guideline to Replace SG6742 with FAN6754A
Introduction
FAN6754A is a highly integrated PWM controller featuring
green mode, frequency hopping, constant power limit, and a
number of protection functions. The green-mode function,
burst-mode function, and a low operating current maximize
the light-load efficiency so that the power supply can meet
stringent standby power regulations. Frequency hopping
reduces the Electro-Magnetic Emission (EMI) by spreading
the frequency spectrum. The constant-power-limit function
minimizes the component stress in abnormal conditions and
helps optimize the power stage design. Protection functions
such as brownout protection, overload / open-loop
protection (OLP), over-voltage protection (OVP), sense pin
short-circuit protection (SSCP), and over-temperature
protection (OTP) are fully integrated to improve the
reliability of the switched-mode power supply without
increasing system cost. This application note explains how
to replace PWM controller SG6742 with FAN6754A. These
two devices have the same pin configuration and direct
replacement can be achieved without PCB layout change.
However, functional improvements to the FAN6754A for
higher efficiency and better performance requires several
external components to 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 less than 100mW standby
power consumption for most designs. The typical
application circuit and internal block diagram are shown in
Figure 1 and Figure 2 , respectively.
Table 1. Comparison of SG6742 and FAN6754A
SG6742
Brownout Protection
Line Voltage Compensation for Pulse-by-
Pulse Current Limit (VLimit-L / VLimit-H)
Sense Pin Short-Circuit Protection
Gate Drive Clamping Voltage
FB Impedance (ZFB)
Operating Current (IDD_OP)
Leading-Edge Blanking Time (tLEB)
Soft-Start
Maximum Duty Cycle
VFB-G/VFB-N for Green Mode
VZDC/VZDCR for Burst Mode
No
Saw-Limit (0.9V / 0.56V)
V <0.15V Longer than 150µs
SENSE
18V
5KΩ
2.7mA
150ns
5ms
65%
2.4V / 3.0V
1.6V / 1.7V
FAN6754A
Line Sensing Using HV Pin
Adjusted by HV Pin (0.46V / 0.39V)
V <0.05V Longer than 120µs
SENSE
13V
15.5KΩ
1.7mA
280ns
8ms
89%
2.3V / 2.8V
2.0V / 2.1V
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 10/8/10
www.fairchildsemi.com