User`s manual

82 RabbitCore RCM2100
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 RCM2100 is typically 16 µA when no other power is sup-
plied. If a 950 mA·h battery is used, the battery can last more than 6 years:
The actual life in your application will depend on the current drawn by components not on
the RCM2100 and the storage capacity of the battery. Note that the shelf life of a lithium
battery is ultimately 10 years.
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 U7, which keeps the 32.768 kHz oscillator working
when the voltage begins to drop.
VRAM and Vcc are nearly equal (<100 mV, typically 10 mV) when power is supplied to
the RCM2100.
Figure C-2 shows the RCM2100 battery-backup circuit.
Figure C-2. RCM2100 Battery-Backup Circuit
950 mA·h
16 µA
------------------------ 6.8 years.=
R7
10 kW
VRAM
VBAT-EXT
External Battery
R42
22 kW
2 kW
R39
RT1
22 kW
mPVBT
Vcc
22 kW
R61
47 kW
U7
pin 5
R60
D8
D12
D11
T
thermistor
C25
1 nF
C9
1 nF