19-6295; Rev 6/12
Converts CMOS RAM into nonvolatile
Unconditionally write protects when VCC is
out of tolerance
Automatically switches to battery when
power fail occurs
Space saving 8-pin PDIP or 8-pin 150 mil SO
Consumes less than 100nA of battery current
VCCI - Input +5 Volt Supply
- RAM Power (VCC) Supply
CEI - Chip Enable Input
NC - No Connection
- Chip Enable Output
- + Battery
The DS1218 is a CMOS circuit which solves the application problems of converting CMOS RAM into
nonvolatile memory. Incoming power is monitored for an out-of-tolerance condition. When such a
condition is detected, the chip enable output is inhibited to accomplish write protection and the battery is
switched on to supply RAM with uninterrupted power. Special circuitry uses a low-leakage CMOS
process which affords precise voltage detection at extremely low battery consumption. The 8-pin package
keeps PC board real estate requirements to a minimum. By combining the DS1218 nonvolatile controller
chip with a full CMOS memory and lithium batteries, 10 years of nonvolatile RAM operation can be
The DS1218 Nonvolatile Controller performs the circuit functions required to battery back-up a RAM.
First, a switch is provided to direct power from the battery or VCCI supply, depending on which is greater.
This switch has a voltage drop of less than 0.2V. The second function which the nonvolatile controller
provides is power-fail detection. The DS1218 constantly monitors the VCC supply. When VCCI falls to
1.26 times the battery voltage, a precision comparator outputs a power-fail detect signal to the chip enable
logic. The third function of write protection is accomplished by holding the chip enable output signal to
within 0.2V of the VCCI or battery supply, when a power-fail condition is detected.
During nominal supply conditions, the chip enable output will follow chip enable input with a maximum
propagation delay of 10 ns.
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