Linear Technology Corporation introduces the LTC3643, 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. 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 3V to 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 2A 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 prioritizing 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, minimizing the size of external components. Low quiescent current Burst Mode operation during regulation maximizes 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.