Power failure protection circuit

In order to prevent data loss caused by sudden power outage or power supply voltage reduction, a power outage protection circuit is used. A voltage comparator LM339 is used in the circuit to monitor the supply voltage. It can be seen from the figure that the reverse input pin 4 is provided by R3 and VL to provide a stable standard voltage of 1.8V. The forward input pin 5 is used as the voltage sampling input terminal of the power supply system. If the voltage of pin 5 is lower than 1.8V, LM339 Pin 2 of the chip output terminal outputs a low level to block the chip select CS of the RAM, and the RAM stops working. The B+ output is a high level to provide the operating voltage to maintain the RAM.