Epson LQ-2550 Technical Manual page 94

2.3.1.5 ST-RAM (1 OA) Battery Backup Circuit
The ST-RAM (1 OA) employes a lithium battery (3.00 to 3.35 VDC) for backup, and is used to maintain
the initial data for the printer mechanism and settings for the control panel
is turned off.
Figure 2-41 shows the ST-RAM (1 OA) battery backup circuit, and Table 2-32 shows the ST-RAM
conditions when the power is turned on or off.
–
When the power is turned on under normal conditions, + 5V is applied to Vcc of the ST-RAM, and the
CPU starts read/write operations. When the power is turned off, and the voltage on the + 5V line drops
to about 3.3V or less, transistors Q 19 and Q20 turn off, and the voltage from the external lithium battery
is applied to the Vcc terminal of the ST-RAM. In this way, the data in the ST-RAM is maintained.
When the voltage from the lithium battery drops, the message "RAM CLEAR" is displayed on the LCD
when the printer power is turned on.
Printer Power
OFF
N O T E : L = O V, H = 5V
Reference: Power consumption at standby: 0.002 [mA] TYP.
CN7
D17
-
--- '
m
- -
3.00 - '-
3.35(v)
Figure 2-41. Battery Backup Circuit
Table 2-32. ST-RAM Conditions with Power ON/OFF
+5 V Line
L
H
m o r e
+ 5
4
Q19
/
-
' ;
u
2 8 Vcc
-
-'
4 ~ND (1
\
777
22)
I
H
2-47
when the printer power
ZD6
2 2
S T - R A M
[v]
S T - R A M M o d e I
2
2.4- 2.7*
+5
REV.-A
3
Standby*
Normal
I