The LTC4121 is a 400mA constant-current/constantvoltage (CC/CV) synchronous step-down battery charger. In addition to CC/CV operation, the LTC4121 regulates its input voltage to a programmable percentage of the input open-circuit voltage. This technique maintains maximum power transfer with high impedance input sources such as solar panels.An external resistor programs the charge current up to 400mA. The LTC4121-4.2 is suitable for charging Li-Ion/ Polymer batteries, while the programmable float voltage of the LTC4121 is suitable for several battery chemistries.The LTC4121 and LTC4121-4.2 include an accurate RUN pin threshold, low voltage battery preconditioning and bad battery fault detection, timer termination, auto-recharge, and NTC temperature qualified charging. TheFAULTpin provides an indication of bad battery or temperature faults.Once charging is terminated, the LTC4121 signals end-ofcharge via theCHRGpin, and enters a low current SLEEP mode. An auto-restart feature starts a new charging cycle if the battery voltage drops by 2.2%.ApplicationsHandheld InstrumentsSolar Powered DevicesIndustrial/Military Sensors and Devices