The schematic diagram shown below is a battery backup regulator circuit, useful for memory or other low power (battery operated) but critical circuit (must continue operation on powerline failure). The one LT020 will not conduct in under line-powered condition, made possible by means of of feedback string’s arrangement. In case of main power failure, the battery-driven LT1020 will turn on and maintain the load, when the line LT1020 go off because the line goes down. Here is the schematic diagram of the circuit:

Battery backup regulator circuit schematic