AN-6755

www.fairchildsemi.com AN-6755 Design Guideline to Replace FAN6753 with FAN6755 Introduction FAN6755 is a highly integrated PWM controller featuring green-mode, frequency hopping, constant power limit, and a number of protection functions. Green mode and burst mode function with a low operating current to maximize the light-load efficiency so 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. Protection functions such as brownout, overload/open-loop (OLP), over-voltage (OVP), and over-temperature (OTP) are fully integrated, which Table 1. Comparison of FAN6753 and FAN6755 FAN6753 HV Pin Input Voltage Brownout Protection Line Voltage Compensation for Pulse-by-Pulse /V ) Current Limit (V limit-L limit-H improves the reliability of switched-mode power supplies (SMPS) without increasing system cost. This application note explains how to replace PWM controller FAN6753 with FAN6755. Only VIN and Latch pins are different; however, some functional improvements have been made to FAN6755 for higher efficiency, lower power consumption, and better performance. Therefore, several external components should be changed accordingly. Table 1 summarizes the differences between these two devices. The operating current is reduced to achieve lower standby power consumption, which allows less than 100mW standby power consumption for most of LCD monitor power supply designs. The typical application circuit and internal block diagram are shown in Figure 1 and Figure 2, respectively. FAN6755 700V Line Sensing Using VIN Pin Adjusted by VIN Pin (0.83V/0.7V) No 700mA 15KΩ 2mA 290ns 7.8V 75% 5.5ms 7-Pin SOP Package 500V No Saw-Limit (0.9V/0.56V) VSENSE