The ISL6256, ISL6256A is a highly integrated battery charger controller for Li-ion/Li-ion polymer batteries. High Efficiency is achieved by a synchronous buck topology and the use of a MOSFET, instead of a diode, for selecting power from the adapter or battery. The low side MOSFET emulates a diode at light loads to improve the light load efficiency and prevent system bus boosting. The constant output voltage can be selected for 2, 3 and 4 series Li-ion cells with 0. 5% accuracy over-temperature. It can also be programmed between 4. 2V + 5%/cell and 4. 2V - 5%/cell to optimize battery capacity. When supplying the load and battery charger simultaneously, the input current limit for the AC adapter is programmable to within 3% accuracy to avoid overloading the AC adapter, and to allow the system to make efficient use of available adapter power for charging. It also has a wide range of programmable charging current. The ISL6256, ISL6256A provides outputs that are used to monitor the current drawn from the AC adapter, and monitor for the presence of an AC adapter. The ISL6256, ISL6256A automatically transitions from regulating current mode to regulating voltage mode. ISL6256, ISL6256A has a feature for automatic power source selection by switching to the battery when the AC adapter is removed or switching to the AC adapter when the AC adapter is available. It also provides a DC adapter monitor to support aircraft power applications with the option of no battery charging.