The LTC3643 from Linear Technology efficiently charges a capacitor array up to 40 V from an input supply between 3 V to 17 V. When the input supply falls below the programmable power-fail threshold, the step-up charger operates in reverse as a synchronous step-down regulator to power the system rail from the backup capacitor during this power interruption/failure condition. When charging the backup capacitor, an external low value sense resistor may be used to maintain an accurate current limit from the input supply or implement PowerPath™ functionality. The step-down converter operates at a 1 MHz switching frequency, allowing small external components to be used. Low quiescent current during regulation maximizes the energy usage from the backup capacitor.