The document introduces CS51021/22/23/24 series fixed frequency PWM current mode controller, which has low startup current (75µA), high frequency operation capability, overvoltage and undervoltage monitoring, programmable dual threshold overcurrent protection, current sense leading edge blanking, current slope compensation, accurate duty cycle control and an externally available 5V reference. The series controller has bidirectional synchronization function, CS51021 and CS51023 provide sleep mode, with maximum IC current consumption of 100µA. The series controller uses a 16-lead narrow body SO package.