X1227_06 Datasheet, PDF (25/28 Pages) Intersil Corporation – 2-Wire™ RTC Real TimeClock/Calendar/ CPU Supervisor with EEPROM

X1227
use the diode to charge a battery (especially lithium
batteries!).
Figure 20. Supercapacitor charging circuit
2.7-5.5V
V_CC
V_back
V_SS
Supercapacitor
Since the battery switchover occurs at Vcc=Vback-
0.1V (see Figure 20), the battery voltage must always
be lower than the Vcc voltage during normal operation
or the battery will be drained. A second consideration
is the trip point setting for the system RESET- func-
tion, known as Vtrip. Vtrip is set at the factory at levels
for systems with either Vcc = 5V or 3.3V operation,
with the following standard options:
V_TRIP = 4.63V ± 3%
V_TRIP = 4.38V ± 3%
V_TRIP = 2.85V ± 3%
V_TRIP = 2.65V ± 3%
The summary of conditions for backup battery opera-
tion is given in Table 8:
Table 8. 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
25
FN8099.2
May 8, 2006