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 Table 1. Comparison of SG6742 and FAN6754A SG6742 Brownout Protection Line Voltage Compensation for Pulse-byPulse 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 V No Saw-Limit (0.9V / 0.56V) SENSE 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. FAN6754A Line Sensing Using HV Pin Adjusted by HV Pin (0.46V / 0.39V) V SENSE