User`s manual

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 ×
VBAT - 0.55 V, or about 2.15 V for a 3 V battery.
2.3.2.1 Power to VRAM Switch
The VRAM switch, shown in Figure 2–7, 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 Q5 is needed to ensure 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 RCM2200 is not resetting (pin 2 on U1 is
high), the /RESET line will be high. This turns on
Q2, causing its collector to go low. This turns on
Q5, allowing VRAM to nearly equal Vcc.
When the RCM2200 is resetting, the /RESET line
will go low. This turns off Q2 and Q5, providing an
isolation between Vcc and VRAM.
The battery backup circuit keeps VRAM from drop-
ping below 2 V.
2.3.2.2 Reset Generator
The RabbitCore RCM2200 uses a reset generator,
U1, 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 RCM2200 has a reset output, pin 9 on header
J5. The reset output may be overdriven with at least
5 mA, essentially turning pin 9 on header J5 into a
reset input, which may be used to reset the RCM2200.
Figure 2–6: RabbitCore RCM2200
Battery Backup Circuit
MMBT5088LT1
Q1
R3
VBAT
R5 R4
VRAM
2
MW
220 kW
1.3 kW
(3 V)
(5 V or
2.2 V)
Figure 2–7: VRAM Switch
FDV302P
Q5
R33
R17
R30
VRAM
VCC
Q2
MMBT3904
22 kW
0 W
/RESET
10 kW