x1205s8izt1 Intersil Corporation, x1205s8izt1 Datasheet - Page 19

• Manufacturer Part Number: x1205s8izt1
• Description: 2-wire? Rtc Real Time Clock/calendar
• Manufacturer: Intersil Corporation

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!).
Table 6. Battery Backup Operation
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
1. Example Application, Vcc=5V, Vback=3.0V
a. Normal Operation
b. Vcc on with no battery
c. Backup Mode
2. Example Application, Vcc=3.3V,Vback=3.0V
a. Normal Operation
b. Vcc on with no battery
c. Backup Mode
d. UNWANTED - Vcc ON, Vback powering
Condition
Condition
19
2.65 - 3.30
0-1.8
0-1.8
5.00
5.00
3.30
3.30
Vcc
Vcc
X1205
1.8-3.0*
Vback
1.8-3.0
Vback
> Vcc
3.00
3.00
Figure 13. Supercapactor charging circuit
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:
0
0
2.7-5.5V
Vtrip
Vtrip
4.38
4.38
4.38
2.65
2.65
2.65
2.65
V
CC
V
up to 3mA
SS
<<1µA
<<1µA
<2µA*
Iback
Iback
<2µA
V
back
0
0
Internal Vcc=Vback
Timekeeping only
Timekeeping only
Supercapacitor
Notes
September 23, 2005
FN8097.2