X1205 Datasheet, PDF (19/22 Pages) Xicor Inc. – Real Time Clock/Calendar

X1205
Depending on the value of supercapacitor used, backup
time can last from a few days to two weeks (with >1F). A
simple silicon or Schottky barrier diode can be used in
series with Vcc to charge the supercapacitor, which is
connected to the Vback pin. Do not use the diode to
charge a battery (especially lithium batteries!).
Figure 13. Supercapactor charging circuit
2.7-5.5V
V_CC
V_back
V_SS
Supercapacitor
Table 6. 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
Since the battery switchover occurs at Vcc=Vback-
0.1V (see Figure 13), the battery voltage must always
be lower than the Vcc voltage during normal operation
or the battery will be drained.
The summary of conditions for backup battery opera-
tion is given in Table 6:
Vback
3.00
0
1.8-3.0
Vtrip
4.38
4.38
4.38
Iback
<<1µA
0
<2µA
Notes
Timekeeping only
2. Example Application, Vcc=3.3V,Vback=3.0V
Condition
Vcc
Vback
a. Normal Operation
3.30
3.00
b. Vcc on with no battery
3.30
0
c. Backup Mode
0-1.8
1.8-3.0*
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
Vtrip
2.65
2.65
2.65
2.65
Iback
<<1µA
0
<2µA*
up to 3mA
Timekeeping only
Internal Vcc=Vback
19
FN8097.2
September 23, 2005