The high-efficiency LTC3128 buck-boost charger developed by Linear Technology integrates active charge balancing for one or two series supercapacitors. Average current limit can be programmed up to 3 A with ±2% accuracy, preventing power-source overload while minimizing capacitor recharge time. Active charge balancing eliminates dissipative external ballast resistors, which balances operation and charging with mismatched capacitors, as well as reduces recharge cycles. A programmable maximum capacitor voltage clamp actively monitors and enforces voltage across each capacitor in the series stack. Thanks to the charger’s low-noise buck-boost topology, the output supercapacitor can be charged whether above or below the input. High-efficiency conversion via low-RDS(ON), low-gate-charge synchronous switches minimizes charging time of storage elements. Input current is controlled over a 0.5- to 3-A programming range, and individual maximum capacitor voltage can be set from 1.6 to 3.0 V. Quiescent current runs less than 1 µA from VOUT in burst-mode operation. Operating temperature ranges from ‒40 to 125°C in either 4- by 5-mm QFN and 24-lead TSSOP packages.