The ADP2125 is a high frequency， step-down， dc-to-dc converter optimized for portable applications in which board area and battery life are critical constraints. Fixed frequency operation at 6 MHz enables the use of tiny ceramic inductors and capacitors. Additionally， the synchronous rectification improves efficiency and results in fewer external components. Over all load currents， the device uses a voltage regulating pulse-width modulation （PWM） mode that maintains a constant frequency with excellent stability and transient response. The ADP2125 is enabled by a 6 MHz to 27 MHz external clock signal applied to the EXTCLK pin. When the external clock is not switching and in a low state （EXTCLK ≤ 0.5 V）， the input is disconnected from the output and draws less than 0.3 μA （typical） from the source. The ADP2125 has 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 ADP2125 is 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.