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2-cell supercapacitor chargers target high peak power, data backup demands for portable applications

March 01, 2010 | Paul Buckley | 222900636
2-cell supercapacitor chargers target high peak power, data backup demands for portable applications Linear Technology Corporation is introducing a family of 2-cell supercapacitor chargers for addressing high peak power, data backup and “dying gasp” needs in portable and data storage applications.
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The LTC4425 employs a linear CC-CV architecture with thermal limiting to charge two supercapacitors in series to a programmable output voltage from a Li-Ion/polymer battery, a USB port, or other 2.7 V to 5.5 V current-limited supply.

The LTC4425 features two operating modes: charge current profile (normal) mode and LDO mode. The devices charge current profile mode charges the top of the supercap stack to the input voltage VIN with a charge current that varies inversely with the input-to-output voltage differential in order to present excessive heating. The LDO mode charges the stack to an externally programmed output voltage with a fixed charge current that is also externally programmable. Charge current is resistor programmable up to 2 A (3 A peak), and each capacitor is protected against overvoltage by internal shunts (2.45 V/2.7 V selectable). The LTC4425s onboard current-limited ideal diode features low 50 mOhm on resistance to prevent back-driving of VIN, making it suitable for a wide variety of high peak power battery and USB-powered equipment, industrial PDAs, portable instruments and monitoring equipment, power meters, supercap backup circuits and PC card/USB modems.

The LTC4425s automatic cell balancing feature maintains equal voltages across both cells, eliminating the need for balancing resistors while protecting each supercapacitor from overvoltage damage while minimizing current drain on the capacitors. The IC operates with a very low 20 uA quiescent current when the output voltage is in regulation and draws only 2 uA in shutdown from either VIN or VOUT, whichever is higher. The basic charging circuit requires only six external components and is highly compact, offered in a tiny 9 mm2 package footprint as well as a leaded package. Other key features include a VIN power fail indicator and continuous monitoring of VIN to VOUT current via the PROG pin. Additional protection features include current and thermal limiting that reduces charge current in cases of excessive temperature.
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