Battery-Backup Circuit - Rabbit RabbitCore RCM4200 User Manual

The drain on the battery by the RCM4200 is typically 7.5 µA when no other power is sup-
plied. If a 165 mA·h battery is used, the battery can last about 2.5 years:
165 mA·h
----------------------- -
7.5 µA
The actual battery life in your application will depend on the current drawn by components
not on the RCM4200 and on the storage capacity of the battery. The RCM4200 does not
drain the battery while it is powered up normally.
Cycle the main power off/on after you install a backup battery for the first time, and when-
ever you replace the battery. This step will minimize the current drawn by the real-time
clock oscillator circuit from the backup battery should the RCM4200 experience a loss of
main power.
NOTE: Remember to cycle the main power off/on any time the RCM4200 is removed
from the Prototyping Board or motherboard since that is where the backup battery
would be located.
Rabbit Semiconductor's Technical Note TN235, External 32.768 kHz Oscillator Circuits,
provides additional information about the current draw by the real-time clock oscillator
circuit.
C.1.2 Battery-Backup Circuit
Figure C-2 shows the battery-backup circuit.
External Battery
VBAT-EXT
The battery-backup circuit serves three purposes:
• It reduces the battery voltage to the SRAM and to the real-time clock, thereby limiting
the current consumed by the real-time clock and lengthening the battery life.
• It ensures that current can flow only out of the battery to prevent charging the battery.
• A voltage, VOSC, is supplied to U6, which keeps the 32.768 kHz oscillator working
when the voltage begins to drop.
120
= 2.5 years.
D1
R16
100 kW
Figure C-2. RCM4200 Backup Battery Circuit
VOSC
VRAM
R19
47 kW
C23
10 nF
RabbitCore RCM4200