Theory of Operation
BATTERY BACKUP
An optional battery backup is available as an on-board 1 Amp-hour
lithium battery. Either or both of the rows of four sockets may be
battery backed. The lithium battery has a long shelf life and will
provide years of backup for low power static RAMs (SRAMs) such as
the Hitachi HM62256LP-15SL. For even longer back-up, use all-
CMOS
SRAMs
such
as
Toshiba
TC55256
PL-12,
NEC
µ
PD44256-12L or Sony CXK58255P-12L which have a 1
µ
A data
retention current. See Table 3-2 for the estimated life of batteries used
with the ZT 8825 with various SRAM chips installed.
Table 3-2
Est. Battery Life @ Max. Standby Current.
CHIPS
CURRENT
ESTIMATED BATTERY LIFE
INSTALLED
DEMAND
(HOURS)
Max.
(
µ
A)
0˚C to +65˚C
Eight 200
µ
A SRAMs
1640
610
Eight 50
µ
A SRAMs
440
2,273
Eight 10
µ
A SRAMs
120
8,333
Eight 1
µ
A SRAMs
48
20,833
Note: It is highly unlikely that the worst case given in Table 3-2 will
ever occur. Typical ZT 8825s with 512K of SRAM draw less than
5
µ
A at +25˚ C, which corresponds to more than 10 years (shelf life
limited).
Convert the number of hours given in Table 3-2 to days, months, or
years based on the number of hours that the main power supply is off
(the battery is the source of power) per day, month, or year.
3-7
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