A low-loss way to implement a”glitch-free” battery backup for a memory or similar circuit is shown on the following schematic diagram. To make LT1020 does not conduct under line-powered conditions, we have to arrange the feedback string. The line LT1020 begins to go off when the lie goes down, which allows the battery-driven LT1020 to turn on and maintain the load.

Battery Backup Regulator circuit schematic

[Linear Technology]