Rabbit BL2020, User Manual

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76 Wildcat (BL2000)
C.2.3 Power to VRAM Switch
The VRAM switch, shown in Figure C-3, allows the battery backup 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 draining the battery.
Figure C-3. VRAM Switch
Field-effect transistor Q9 is needed to provide a very small voltage drop between Vcc and
VRAM (<100 mV, typically 10 mV) so that the board components powered by Vcc will
not have a significantly different voltage than VRAM.
When the BL2000 is not in reset, the /RESET line will be high. This turns on Q8, causing
its collector to go low. This turns on Q9, allowing VRAM to nearly equal Vcc.
When the BL2000 is in reset, the
/RESET
line will go low. This turns off Q8 and Q9, pro-
viding an isolation between Vcc and VRAM.
FDV302P
Q9
R99
R104
R93
VRAM VCC
Q8
MMBT3904
22 k W
0 W
/RESET
11 k W