Linear Technology LTC3625 and LTC3625-1 two-cell supercapacitor chargers

Linear Technology has introduced the LTC3625 and LTC3625-1, the latest in a family of two-cell supercapacitor chargers for addressing high peak power, data backup and ‘dying gasp’ needs in portable and data storage applications.

Their high-efficiency switchmode topology includes an internal buck converter between V_IN and the series capacitor mid-point V_MID to regulate the voltage on the bottom capacitor, as well as an internal boost converter between V_MID and V_OUT to regulate the voltage across the top capacitor.

The devices can charge two supercapacitors in series from a 2.7 to 5.5 V current-limited supply to a pin-selectable output voltage (4.8/5.3 for LTC3625 or 4/4.5 for LTC3625-1).

The ICs are optimised to provide fast capacitor charge times with high efficiency. The combination of high-efficiency, high-charge current, low quiescent current and low minimum external parts count make the LTC3625/-1 suitable for small form factor backup or high peak power systems, USB-powered equipment, industrial PDAs, portable instruments and monitoring equipment, power meters, supercap backup circuits and PC card/USB modems.

The output current of the LTC3625/-1’s buck converter is user programmable and the input current of the boost converter is set at 2 A (typical). The resulting maximum charge current is 1 A continuous with two inductors and 500 mA with a single inductor configuration.

Each capacitor is protected against overvoltage by internal voltage clamps (2.4/2.65 selectable for LTC3625 or 2.0/2.25 selectable for the LTC3625-1). The LTC3625/-1’s automatic cell balancing feature maintains about equal voltages across both cells, eliminating the need for balancing resistors while protecting each supercapacitor from overvoltage damage.

The devices operate with a low 50 µA quiescent current when the output voltage is in regulation. When the input supply is removed or the EN pin is low, the LTC3625/-1 automatically enters a low current state, drawing less than 1 µA from the supercapacitors. The basic charging circuit requires only three external components (one inductor, one bypass capacitor at V_IN and one programming resistor) and is compact.