The LM3686 is a step-down DC-DC converter with one integrated very low dropout linear regulator and a low noise linear regulator optimized for powering ultra-low voltage circuits from a single Li-Ion cell or 3 cell NiMH/NiCd batteries. It provides three outputs with combined load current up to 900 mA over an input voltage range from 2.7V to 5.5V. The device offers superior features and performance for many applications. Automatic intelligent switching between PWM low-noise and PFM low-current mode offers improved system control. During full-power operation, a fixed-frequency 3 MHz (typ.), PWM mode drives loads from ~70 mA to 600 mA max. Hysteretic PFM mode extends the battery life through reduction of the quiescent current to 28 A (typ.) at light load and system standby. Internal synchronous rectification provides high efficiency. Three enable pins allow the separate operation of either the DC-DC, post-regulation linear regulator or the linear regulator alone. If the DC-DC is not enabled during startup of the postregulation linear regulator, a parallel small pass transistor supplies the linear regulator from VBATT with maximal 50 mA. In the combined operation where both enables are raised together, the small pass transistor is deactivated and the big pass transistor provides 350 mA output current. In shutdown mode (Enable pins pulled low), the device turns off and reduces battery consumption to 2.5 A (typ.). The LM3686 is available in a 12-bump micro SMD package. A high-switching frequency of 3 MHz (typ.) allows the use of a few tiny surface-mount components. Only six external surface-mount components, an inductor and five ceramic capacitors, are required to establish a 15.66 mm2 total solution size.