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C.2.3 Power to VRAM Switch
The VRAM switch, shown in Figure C-2, allows the battery backup to provide power
when the external power goes off. The switch provides an isolation between +5 V and the
battery when +5 V goes low. This prevents the +5 V line from draining the battery.
R38
+5 V
VRAM
(not installed)
0 W
Q6
FDV302P
R37
10 k W
/RES
R36
22 k W
Q7
MMBT3904
Figure C-2. VRAM Switch
Transistor Q6 is needed to provide a very small voltage drop between +5 V and VRAM
(<100 mV, typically 10 mV) so that the processor lines powered by +5 V will not have a
significantly different voltage than VRAM.
When the CPU Card is not resetting (pin 2 on U4 is high), the /RES line will be high. This
turns on Q6, causing its collector to go low. This turns on Q7, allowing VRAM to nearly
equal +5 V.
When the CPU Card is resetting, the /RES line will go low. This turns off Q6 and Q7, pro-
viding an isolation between +5 V and VRAM.
The battery-backup circuit keeps VRAM from dropping below 2 V.
168
Smart Star (SR9000)
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