User`s manual

User’s Manual 43
C.1.2 Battery Backup Circuit
The battery-backup circuit serves two purposes:
• It reduces the battery voltage to the real-time clock, thereby reducing the current con-
sumed 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.
Figure C-2 shows the RabbitCore 2000 battery backup circuitry.
Figure C-2. RabbitCore 2000 Battery Backup Circuit
Resistor R12, shown in Figure C-2, is typically not stuffed on the RabbitCore 2000.
VRAM and Vcc are nearly equal (<100 mV, typically 10 mV) when power is supplied to
the RabbitCore 2000. R14 prevents any catastrophic failure of Q12 by limiting current
from the customer-supplied battery.
Resistors R11 and R20 make up a voltage divider between the battery voltage and the tem-
perature-compensation voltage at the anode of diode D12. This voltage divider biases the
base of Q12 to about 0.9 × VBAT. V
BE
on Q12 is about 0.55 V. Therefore, VRAM is about
0.9 × VBAT - 0.55 V, or about 2.15 V for a 3 V battery.
MMBT5088LT1
Q12
R12
R20
R13
VBAT
R14 R11
D12
D11
D10
R19
0 Ω
R18
0 Ω
R17
0 Ω
VRAM
0 Ω
4.3 MΩ
2 MΩ
220 kΩ
1.3 kΩ