X1288 Datasheet, PDF (25/27 Pages) Xicor Inc. – 2-Wire RTC Real Time Clock/Calendar/CPU Supervisor with EEPROM

X1288
The summary of conditions for backup battery operation is given in Table 9:
Table 9. Battery Backup Operation
1. Example Application, Vcc=5V, Vback=3.0V
Condition
Vcc
a. Normal Operation
5.00
b. Vcc on with no battery
5.00
c. Backup Mode
0–1.8
Vback
3.00
0
1.8-3.0
Vtrip
4.38
4.38
4.38
Iback
<<1µA
0
<2µA
Reset
H
H
L
Notes
Timekeeping
only
2. Example Application, Vcc=3.3V,Vback=3.0V
Condition
Vcc
a. Normal Operation
3.30
b. Vcc on with no battery
3.30
c. Backup Mode
0–1.8
Vback
3.00
0
1.8–3.0*
Vtrip
2.65
2.65
2.65
d. UNWANTED - Vcc ON, Vback
powering
2.65 - 3.30 > Vcc
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
2.65
Iback
<<1µA
0
<2µA*
Reset
H
H
L
up to 3mA
H
Timekeeping
only
Internal
Vcc=Vback
Referring to Figure 20, 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.
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.
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 9). This will cause
the battery to drain quickly as serial bus communica-
tion and non-volatile writes will require higher supplier
current.
PERFORMANCE DATA
I_BACK Performance
I_BACK vs. Temperature
Multi-Lot Process Variation Data
1.4
3.3V
1.2
1.0
1.8V
0.8
0.6
0.4
0.2
0
-40
25
60
85
Temperature °C
25
FN8102.3
April 14, 2006