Thanks to its high-efficiency, synchronous step-down topology, the LTC4121 constant-current/constant-voltage 400-mA battery charger accepts a 4.4- to 40-V input. The charger, developed by Linear Tech, maintains a programmable 3.5- o 18-V battery charge voltage range with tight +1% accuracy, suiting it for battery chemistries such as lithium-ion, LiFePO4, and lead-acid. In addition, a fixed-output, float-voltage version—the LTC4141-4.2—will fit 1S lithium-ion battery packs. The synchronous architecture enables charging efficiency up to 94%, as well as low-dropout startup and operation. The LTC4121 can regulate its input voltage to a programmable percentage of the input open-circuit voltage to accommodate high-impedance input sources (e.g., solar panels). If the input voltage falls below the user-defined percentage of the open-circuit voltage, it automatically reduces charge current. Programmable charge current ranges from 50 to 400 mA (there’s also low-voltage battery preconditioning). A recharge function starts a new charging cycle if the battery voltage drops 2.2% below the programmed charge voltage. Packaging is a 16-pin, 3- by 3-mm QFN; operating temperature ranges from ‒40 to 125°C.