Diode Battery Back-Up Switch
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
This article describes a circuit arrangement in a personal computer (PC) system which switches in a back-up battery for the real time clock and oscillator circuits on system power-off and isolates the back-up battery on system power-up. The diode battery back-up circuit disclosed herein is shown in the drawing. When the system is on, the zener diode (D5) sets the reference voltage at node A to a constant 6.8 volts. The voltage at node B, the positive supply voltage (VDD) for the real-time clock and oscillator circuits is 6.8 volts minus the voltage drop across diodes D1 and D2. Since the forward bias voltage (VF) for diodes D1 and D2 is 0.5 volt, the voltage at node B is 5.8 volts. With node B at 5.