TECHNICAL BULLETIN Memory Back-up
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MEMORY BACK-UP
The rapid advancement of CMOS technology has permitted development of a new generation of electronic equipment with memory functions and a real time clock/calendar. These functions require reliable standby power in order to maintain the memory and time in the event of mainline power shutdown. DURACELL Li/MnO₂ batteries are particularly attractive as back-up power supplies because of their high energy density, excellent shelf life characteristics, safety, high reliability, and resistance against leakage.

Figure 7.1.: Service life expectancy of the DURACELL DL2/3A under continuous microampere drains.



For memory back-up applications, the combination of consumer replaceable DL2/3A battery and standard 2/3A-size battery holder is an ideal solution when ease of battery replacement is important.
The following figures show typical circuits for a battery backed CMOS RAM.

Figure 7.2.: Memory back-up circuit using two diodes.



Figure 7.3.: Memory back-up circuit using diode and resistor.



A blocking diode is used to protect the primary Li/MnO₂ cell against charging by the main power source. This blocking diode should have the features of a low voltage drop in the forward direction to minimize the loss of battery back-up voltage, and a low leakage current in the reverse direction to minimize the charging current. The diode used should be capable of controlling the leakage current according to the values given in the following table.

DURACELL Li/MnO₂ batteries are recognized for user replacement under the Component Program of Underwriters Laboratories.

Table:  Maximum permissible reverse current. Intentional charging of primary Li/MnO₂ cells is not recommended.