Battery-Backup Circuit - Rabbit RabbitCore RCM3305 User Manual

A lithium battery with a nominal voltage of 3 V and a minimum capacity of 165 mA·h is
recommended. A lithium battery is strongly recommended because of its nearly constant
nominal voltage over most of its life.
The drain on the battery by the RCM3305/RCM3315 is typically 6 µA when no other power
is supplied. If a 165 mA·h battery is used, the battery can last about 3 years:
165 mA·h
----------------------- -
6 µA
The RCM3305/RCM3315 does not drain the battery while it is powered up normally.
Cycle the main power off/on on the RCM3305/RCM3315 after you install a backup battery
for the first time, and whenever you replace the battery. This step will minimize the current
drawn by the real-time clock oscillator circuit from the backup battery should the
RCM3305/RCM3315 experience a loss of main power.
NOTE: Remember to cycle the main power off/on any time the RCM3305/RCM3315 is
removed from the Protoyping Board or motherboard since that is where the backup
battery would be located.
Rabbit's Technical Note TN235, External 32.768 kHz Oscillator Circuits, provides addi-
tional information about the current draw by the the real-time clock oscillator circuit.
D.1.2 Battery-Backup Circuit
Figure D-2 shows the battery-backup circuit.
External Battery
VBAT-EXT
Figure D-2. RCM33305/RCM3315 Backup Battery Circuit
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 U1, which keeps the 32.768 kHz oscillator working
when the voltage begins to drop.
136
= 3.1 years.
VRAM
D1
R46
150 kW
VOSC
R7
R3
100 W 47 kW
C2
100 nF
RabbitCore RCM3305/RCM3315
C5
10 nF