The ADP2126/ADP2127 are high frequency， step-down， dc-todc converters optimized for portable applications in which board area and battery life are critical constraints. The fixed 6 MHz operating frequency enables the use of tiny ceramic inductors and capacitors and the regulators use spread spectrum frequency modulation to reduce EMI. Additionally， synchronous rectification improves efficiency and results in fewer external components. At high load currents， the ADP2126/ADP2127 use a voltage regulating pulse-width modulation （PWM） mode that maintains a constant frequency with excellent stability and transient response. Light load operation is determined by the state of the MODE pin. In forced PWM mode， the converter continues operating in PWM for light loads. Under light load conditions in auto mode， the ADP2126/ADP2127 automatically enter a power-saving mode， which uses pulse frequency modulation （PFM） to reduce the effective switching frequency， thus ensuring the longest battery life in portable applications. The ADP2126/ADP2127 are enabled by a 6 MHz to 27 MHz external clock signal applied to the EXTCLK pin. Certain models can also be enabled with a logic high signal. When the external clock is not switching and in a low logic state， the ADP2126/ADP2127 stop regulating and shut down to draw less than 0.3 μA （typical） from the source. The ADP2126/ADP2127 have an input voltage range of 2.1 V to 5.5 V， allowing the use of single Li+/Li polymer cell， three-cell alkaline， NiMH cell， and other standard power sources. The ADP2126/ADP2127 are internally compensated to minimize external components and can source up to 500 mA. Other key features， such as cycle-by-cycle peak current limit， soft start， undervoltage lockout （UVLO）， output-to-ground short-circuit protection， and thermal shutdown provide protection for internal and external circuit components.