The ADP2109 is a high efficiency， low quiescent current stepdown dc-to-dc converter with an internal discharge switch that allows automatic discharge of the output capacitor in an ultrasmall 5-ball WLCSP package. The total solution requires only three tiny external components. It uses a proprietary high speed current mode and constant frequency pulse-width modulation （PWM） control scheme for excellent stability， and transient response. To ensure the longest battery life in portable applications， the ADP2109 has a power save mode that reduces the switching frequency under light load conditions. The ADP2109 runs on input voltages of 2.3 V to 5.5 V， which allow for single lithium or lithium polymer cell， multiple alkaline or NiMH cells， PCMCIA， USB， and other standard power sources. The maximum load current of 600 mA is achievable across the input voltage range. The ADP2109 is available in fixed output voltages of 1.8 V， 1.5 V， 1.2 V， and 1.0 V. All versions include an internal power switch and synchronous rectifier for minimal external part count and high efficiency. The ADP2109 has an internal soft start and internal compensation. During logic-controlled shutdown， the input is disconnected from the output and the ADP2109 draws less than 1 μA from the input source. Other key features include undervoltage lockout to prevent deep battery discharge and soft start to prevent input current overshoot at startup. The ADP2109 is available in a 5-ball WLCSP. A similar converter， the ADP2108， provides the same features and operations as the ADP2109 without the discharge switch and is available in both WLCSP and TSOT packages with additional output voltages