The ADP2138 and ADP2139 are high efficiency， low quiescent current， synchronous step-down dc-to-dc converters. The ADP2139 has the additional feature of an internal discharge switch. The total solution requires only three tiny external components. When the MODE pin is set high， the buck regulator operates in forced PWM mode， which provides low peak-to-peak ripple for power supply noise sensitive loads at the expense of light load efficiency. When the MODE pin is set low， the buck regulator automatically switches operating modes， depending on the load current level. At higher output loads， the buck regulator operates in PWM mode. When the load current falls below a predefined threshold， the regulator operates in power save mode （PSM）， improving light load efficiency. The ADP2138/ADP2139 operate on input voltages of 2.3 V to 5.5 V， which allows for single lithium or lithium polymer cell， multiple alkaline or NiMH cell， PCMCIA， USB， and other standard power sources. The maximum load current of 800 mA is achievable across the input voltage range. The ADP2138/ADP2139 are available in fixed output voltages of 3.3 V， 3.0 V， 2.8 V， 2.5 V， 1.8 V， 1.5 V， 1.2 V， 1.0 V， and 0.8 V. All versions include an internal power switch and synchronous rectifier for minimal external part count and high efficiency. The ADP2138/ADP2139 have internal softstart and they are internally compensated. During logic controlled shutdown， the input is disconnected from the output and the ADP2138/ADP2139 draw 0.2 μA （typical） 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 ADP2138/ADP2139 are available in a 6-ball wafer level chip scale package （WLCSP）.