Power Supply Isolator for RAM Back-Up Battery
Original Publication Date: 1983-Nov-01
Included in the Prior Art Database: 2005-Feb-07
In a random-access memory (RAM) system shown in Fig. 1 and Fig. 3, the battery must supply auxiliary power to the RAM when the 5 V supply falls below a critical level. This protects the contents of the RAM during loss of main power. However, the battery must not be loaded by other 5 V circuitry, which would shorten battery life. Automatic switching is provided to battery power when the 5 V supply drops, which disconnects other loads. Low series loss is provided when the 5 V supply is up and reverse current flow into the battery is prevented. This is accomplished using a power metal oxide semiconductor field-effect transistor (MOSFET) with small "on" resistance. In Fig. 1, T1 is a series P channel MOSFET switch. The intrinsic drain-to-source diode is used to facilitate switching as the 5 V supply, VS, comes up.