X1227V8T1
| Model | X1227V8T1 |
| Description | REAL TIME CLOCK, PDSO8, PLASTIC, TSSOP-8 |
| PDF file | Total 28 pages (File size: 422K) |
| Chip Manufacturer | RENESAS |
X1227
is to use a supercapacitor for applications where Vcc
may disappear intermittently for short periods of time.
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 superca-
pacitor, which is connected to the Vback pin. Do not
use the diode to charge a battery (especially lithium
batteries!).
Figure 20. Supercapacitor charging circuit
2.7-5.5V
V
CC
V
back
Supercapacitor
V
SS
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
a. Normal Operation
b. Vcc on with no battery
c. Backup Mode
Vcc
5.00
5.00
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
a. Normal Operation
b. Vcc on with no battery
c. Backup Mode
d. UNWANTED - Vcc ON, Vback
powering
Vcc
3.30
3.30
0-1.8
2.65 - 3.30
Vback
3.00
0
1.8-3.0*
> Vcc
Vtrip
2.65
2.65
2.65
2.65
Iback
<<1µA
0
<2µA*
up to 3mA
Reset
H
H
L
H
Timekeeping
only
Internal
Vcc = Vback
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
25
FN8099.1
September 15, 2005
