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Power Semiconductors

Bidirectional Boost Capacitor Charger Provides System Power Backup

07 January 2016

The LTC3643 from Linear Technology Corp. is a bidirectional, high voltage boost capacitor charger that automatically converts to a buck regulator for system backup. The proprietary, single-inductor topology with integrated PowerPath functionality does the work of two separate switching regulators, reducing size, cost and complexity.

LTC3643 boost capacitor charger (Source: Linear Technology).LTC3643 boost capacitor charger (Source: Linear Technology).

The LTC3643 operates in two modes: boost charge mode and buck backup mode. The charging mode is designed to efficiently charge an electrolytic capacitor array up to 40 V with an internal switch current rating of 2A from an input supply between 3 V to 17 V. In backup mode, when the input supply falls below the programmable power-fail (PFI) threshold, the step-up charger operates in reverse as a synchronous step-down regulator to power and hold up the system rail from the backup capacitor.

During backup, the current limit can be programmed from 2 A to 4 A, making this device ideal for high energy, relatively short duration backup capacitor systems, power failure backup systems, solid-state drives and battery stack charging applications.

When charging the backup capacitor, an external low-value sense resistor can be used by the LTC3643 to maintain an accurate current limit from the input supply while prioritizing power delivery to the system load. Input current limit can be programmed with the 50 mV threshold across a current sense resistor, preventing system power source by reducing capacitor charging current. The step-down converter operates at a 1 MHz frequency, minimizing the size of external components. Low quiescent current Burst Mode operation during regulation maximizes the energy usage from the backup capacitor.

Other features include an integrated power N-channel MOSFET for output/CAP disconnect, ± 1percent reference accuracy for system voltage regulation, and indicator outputs for charge status and input power fail.

The LTC3643 is housed in a thermally enhanced 24-lead 3 mm x 5 mm QFN package, featuring E- and I-grade operation from -40°C to 125°C. Pricing for the E-grade device starts at $4.45 each in 1,000-piece quantities.



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