SD4840/4841/4842/4843/4844 is a current mode PWM controller with low standby power and low start current for power switch. In standby mode, the circuit enters burst mode to reduce the standby power dissipation. The switch frequency is 67KHz with ±2.5 KHz jitter frequency for low EMI. The stress on transformer during power on is reduced by the builtin 15ms soft start circuit to avoid the saturation of transformer. SD4840/4841/4842/4843/4844 includes under voltage lock-out, over voltage protection, leading edge blanking, over current protection and the temperature protection. The circuit will restart automatically until the system is normal after the protection is active.