Bidirectional backup supply provides continuous power during outages

The LTC3643 operates in two modes - boost charge mode and buck backup mode. The charging mode efficiently charges an electrolytic capacitor array up to 40V with an internal switch current rating of 2A from an input supply between 3 and 17V. 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 to 4A, 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 prioritising power delivery to the system load. Input current limit can be programmed with the 50mV threshold across a current sense resistor, preventing system power source by reducing capacitor charging current. The step-down converter operates at a 1MHz frequency, minimising the size of external components. Low quiescent current Burst Mode operation during regulation maximises the energy usage from the backup capacitor.

The LTC3643 provides ideal diode operation at the input supply by providing the gate drive signal to an external PMOS switch. This enables efficient power delivery while providing complete isolation between the input supply and the system load during backup mode. Other features include an integrated power N-channel MOSFET for output/CAP disconnect, ±1% reference accuracy for system voltage regulation, and indicator outputs for charge status and input power fail.

The LTC3643 is housed in compact, thermally enhanced 24-lead 3x5mm QFN package, featuring E- and I-grade operation from -40 to +125°C. Pricing for the E-grade device starts at $4.45 each in 1,000-piece quantities, and all are available from stock.