The LX1570/71 controller IC family is designed to provide all the control functions of an isolated auxiliary secondary side regulator or secondary power supply. Auxiliary or used in secondary side controllers for a variety of applications, including multi-output power-offs, typically found in desktop computers, and telecom applications. Although they can be used in all secondary output applications requiring precise regulation, they are primarily optimized to provide more output than the 3A standard three-terminal regulators lack the desired efficiency. For these applications, MAG amplifier regulation has traditionally been used. However, MAG amps have several disadvantages. First, because they withstand the maximum input voltage under short-circuit conditions, they are "over-engineered," typically supplied by a factor of 2, increasing power cost and size. Second, MAG amplifiers are inherently leading-edge modulation, so they can only approach the core of the MAG-ampere loop characteristics of a certain maximum duty cycle, finite minimum delay and magnetic wave black. The magnitude of this force increases the main transformer as well as the output inductor, resulting in higher overall system cost. The LX1570/71 eliminates all the shortcomings of the MAG amplifier approach as well as improving system performance and reducing overall system cost. The LX1570/71 is a current-mode controller IC that controls the duty cycle of a switch series auxiliary power output in step-down-derived power transformer-derived applications such as forward or bridge topologies. It provides features such as 100% duty cycle operation for maximum energy transfer, pulsed and discontinuous current limiting, long off-time between cycles for reduced power consumption, high frequency for smaller magnetic components, soft-start operation, and excellent current mode control dynamic response.