Power To Vram Switch - Z-World Wildcat BL2000 User Manual

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.
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
its collector to go low. This turns on Q9, allowing VRAM to nearly equal Vcc.
When the BL2000 is in reset, the
viding an isolation between Vcc and VRAM.
74
VCC
FDV302P
R104
/RESET
22 kW
Figure C-3. VRAM Switch
/RESET
/RESET line will go low. This turns off Q8 and Q9, pro-
VRAM
R99
0 W
Q9
R93
11 kW
Q8
MMBT3904
line will be high. This turns on Q8, causing
Wildcat (BL2000)