Diodes' AP3984 primary side power switcher has a built in BJT and regulates the output voltage and current in the primary side by piece-wise pulse frequency modulation (p-PFM) and primary current peak amplitude modulation (AM) in discontinuous conduction mode (DCM). The system operating frequency reduces linearly from heavy load to light load in each interval of the p-PFM, and operating frequency is fixed at medium load by varying primary current peak amplitude. The AP3984 has good transient characteristics in combination with the secondary side IC. Typically, a minimal voltage of 4.3 V at PCB side can be achieved for a dynamic test of a 5 V application system. The AP3984 provides an operating frequency dithering function to improve EMC performance of power supply. The AP3984 also has a programmable cable voltage drop compensation function by external resistor and is packaged in SO-7.