RabbitCore RCM2200, Руководство пользователя

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2–8 Hardware Reference RabbitCore RCM2200
The battery-backup circuit serves two purposes:
• It reduces the battery voltage to the real-time
clock, thereby reducing the current consumed
by the real-time clock and lengthening the bat-
tery life.
• It ensures that current can flow only out of the
battery to prevent charging the battery.
Figure 2–6 shows the RabbitCore 2000 battery
backup circuit.
It is important not to charge a lithium battery with
any appreciable current. Resistor R5, shown in
Figure 2–6, prevents any catastrophic failure of Q1
by limiting current to the customer-supplied battery.
VRAM and Vcc are nearly equal (<100 mV, typi-
cally 10 mV) when power is supplied to the Rabbit-
Core RCM2200.
Resistors R3 and R4 make up a voltage divider that
biases the base of Q1 to about 0.9 × VBAT. V
BE
on
Q1 is about 0.55 V. Therefore, VRAM is about 0.9 ×