X1228
Referring to Figure 16, Vtrip applies to the “Internal
Vcc” node which powers the entire device. This means
that if Vcc is powered down and the battery voltage at
Vback is higher than the Vtrip voltage, then the entire
chip will be running from the battery. If Vback falls to
lower than Vtrip, then the chip shuts down and all out-
puts are disabled except for the oscillator and time-
keeping circuitry. The fact that the chip can be powered
from Vback is not necessarily an issue since standby
current for the RTC devices is <2µA for this mode
(called “main timekeeping current” in the data sheet).
Only when the serial interface is active is there an
increase in supply current, and with Vcc powered
down, the serial interface will most likely be inactive.
PERFORMANCE DATA
Performance
I
BACK
I
vs. Temperature
BACK
Multi-Lot Process Variation Data
1.4
3.3V
1.2
1.0
0.8
0.6
0.4
0.2
0
1.8V
One way to prevent operation in battery backup mode
above the Vtrip level is to add a diode drop (silicon
diode preferred) to the battery to insure it is below
Vtrip. This will also provide reverse leakage protection
which may be needed to get safety agency approval.
-40
25
60
85
Temperature °C
One mode that should always be avoided is the opera-
tion of the RTC device with Vback greater than both
Vcc and Vtrip (Condition 2d in Table 8). This will cause
the battery to drain quickly as serial bus communica-
tion and non-volatile writes will require higher supplier
current.
Characteristics subject to change without notice. 27 of 31
REV 1.3 3/24/04
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