The IC normally powers the load from the higher priority main supply such as a wall adapter or battery, switching to the backup supply—typically a battery or large-value capacitor—during primary brownout or power loss conditions. With up to 18V operating capability, the LTC4420 accommodates a wide range of power sources such as wall adapters, USB ports, supercapacitors, and stacked batteries with Li-Ion, alkaline or NiMH cells. By reducing device operating current to 3.6 µA and the backup supply’s standby current drain to 0.32 μA, the LTC4420 extends run time in portable battery-operated systems.
Unlike other solutions, the LTC4420 does not require the backup supply voltage to be lower than the main supply voltage. When the backup supply voltage is higher than the main supply voltage, a prioritizer is a better solution than a simple diode-OR as the prioritizer does not necessarily draw current from the highest voltage supply. Internal 500 mA (min) current-limited switches multiplex from the primary to the backup supply when the primary voltage falls below the adjustable switchover threshold. Break-before-make fast switch control blocks reverse and cross conduction currents while minimizing output voltage droop. To reduce quiescent current, the backup supply’s resistive divider is connected to a strobed ground with a duty cycle that is adjustable down to a low 0.0015%, and an optional undervoltage disconnect prevents deeply discharged batteries. A ‘freshness seal’ mode further lowers standby current to 0.12 µA, minimizing battery discharge during product storage and shipment. Reversed-battery voltages up to −15V are blocked from propagating to the load.
Specified over the 0°C to 70°C commercial and −40°C to 85°C industrial temperature ranges, the LTC4420 comes in thermally enhanced 12-pin MSOP and 3 x 3 mm DFN packages, with pricing from $2.95 (1000).
Linear Technology; www.linear.com/product/LTC4420