User`s manual

User’s Manual Hardware Reference 2–11
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 2–8 shows the RabbitCore 2000 battery
backup circuit.
Resistor R7, shown in Figure 2–8, prevents any cat-
astrophic failure of Q1 by limiting current from the
customer-supplied battery. VRAM and Vcc are
nearly equal (<100 mV, typically 10 mV) when
power is supplied to the RabbitCore RCM2100.
Resistors R8 and R9 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 ×
VBAT - 0.55 V, or about 2.15 V for a 3 V battery.
VRAM is also available on pin 29 of header J2
to facilitate battery backup of the external cir-
cuit. Note that the recommended maximum
external current draw from VRAM is 100 µA,
and new battery-life calculations should be
done to take external loading into account.
2.4.2.1 Power to VRAM Switch
The VRAM switch, shown in Figure 2–9, allows a
customer-supplied external battery to provide power
when the external power goes off. The switch pro-
vides an isolation between Vcc and the battery when
Vcc goes low. This prevents the Vcc line from
draining the battery.
Transistor Q2 is needed to provide a very small volt-
age drop between Vcc and VRAM (<100 mV, typi-
cally 10 mV) so that the processor lines powered by
Vcc will not have a significantly different voltage
than VRAM.
When the RabbitCore RCM2100 is not resetting
(pin 2 on U3 is high), the /RESET line will be high.
This turns on Q3, causing its collector to go low.
This turns on Q2, allowing VRAM to nearly equal
Vcc.
When the RabbitCore RCM2100 is resetting, the
/RESET line will go low. This turns off Q2 and Q3,
providing an isolation between Vcc and VRAM.
The battery backup circuit keeps VRAM from drop-
ping below 2 V.
2.4.2.2 Reset Generator
The RabbitCore RCM2100 uses a reset generator,
U2, to reset the Rabbit 2000 microprocessor when
the voltage drops below the voltage necessary for
reliable operation. The reset occurs between 4.50 V
and 4.75 V, typically 4.63 V. The RabbitCore
Figure 2–8: RabbitCore RCM2100
Battery Backup Circuit
MMBT5088LT1
Q1
R8
C6
VBAT
R7 R9
VRAM
100 nF
2 MW
220 kW
1.3 kW
Figure 2–9: VRAM Switch
FDV302P
Q2
R12
R15
R14
VRAM
VCC
Q3
MMBT3904
22 kW
0 W
/RESET
10 kW