User`s manual

44 RabbitCore 2000
These voltages vary with temperature. VRAM varies the least because temperature-com-
pensation diodes D10–D12 will offset the variation with temperature of Q12’s V
BE
. R17–
R19 may be stuffed instead of the corresponding D10–D12 to provide the optimum tem-
perature compensation.
Resistor R13 provides a minimum load to the regulator circuit.
VRAM is also available on pin 34 of header J2 to facilitate battery backup of the external
circuit. Note that the recommended minimum resistive load at VRAM is 100 kΩ, and new
battery life calculations should be done to take external loading into account.
C.1.3 Power to VRAM Switch
The VRAM switch, shown in Figure C-3, allows a customer-supplied external battery to
provide power when the external power goes off. The switch provides an isolation
between Vcc and the battery when Vcc goes low. This prevents the Vcc line from drain-
ing the battery.
Figure C-3. VRAM Switch
Transistor Q11 is needed to provide a very small voltage drop between Vcc and VRAM
(<100 mV, typically 10 mV) so that the processor lines powered by Vcc will not have a
significantly different voltage than VRAM.
When the RabbitCore 2000 is not resetting (pin 2 on U10 is high), the /RES_OUT line will
be high. This turns on Q10, causing its collector to go low. This turns on Q11, allowing
VRAM to nearly equal Vcc.
When the RabbitCore 2000 is resetting, the /RES_OUT line will go low. This turns off
Q10 and Q11, providing an isolation between Vcc and VRAM.
The battery backup circuit keeps VRAM from dropping below 2 V.
FDV302P
Q11
R10
R15
R16
VRAM
VCC
Q10
MMBT3904
22 kΩ
0 Ω
/RES_OUT
10 kΩ