A flexible and cost-efficient controller for high-performance flyback power converters can operate in either the fixed-frequency (FF) or the quasi-resonant (QR) mode, the two principal operating modes used in flyback switched-mode power supplies (SMPS). The L6566 controller can select different modes by connection of a single pin, thereby enabling manufacturers to stock and qualify a single component to meet the requirements of different applications. In addition, the chip integrates many protection capabilities commonly implemented with external components, as well as energy-saving features that help the designer meet severe energy regulations. The selectable multimode controller is suitable in applications such as flyback ac/dc adapters, TV and LCD monitors, small-sized LCD TVs, high-end chargers, and in DVD players and set-top boxes. Under very light load conditions, in both operating modes, the chip enters a controlled burst-mode state at low frequency (a few hundred hertz) with constant peak current, in order to minimize total power requirements and to comply with worldwide energy-saving recommendations. The controller’s quiescent current (less than 3 mA) and onboard nondissipative start-up circuit—a product of the company’s high-voltage process technology—further improve the overall efficiency.