25
FN8099.2
May 8, 2006
use the diode to charge a battery (especially lithium
batteries!).
Figure 20. Supercapacitor charging circuit
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
*since Vback>2.65V is higher than Vtrip, the battery is powering the entire device
2.7-5.5V
Supercapacitor
V
SS
V
CC
V
back
1. Example Application, Vcc = 5V, Vback = 3.0V
Condition Vcc Vback Vtrip Iback Reset Notes
a. Normal Operation 5.00 3.00 4.38 <<1µA H
b. Vcc on with no battery 5.00 0 4.38 0 H
c. Backup Mode 0-1.8 1.8-3.0 4.38 <2µA L Timekeeping
only
2. Example Application, Vcc=3.3V,Vback=3.0V
Condition Vcc Vback Vtrip Iback Reset
a. Normal Operation 3.30 3.00 2.65 <<1µA H
b. Vcc on with no battery 3.30 0 2.65 0 H
c. Backup Mode 0-1.8 1.8-3.0* 2.65 <2µA* L Timekeeping
only
d. UNWANTED - Vcc ON, Vback
powering
2.65 - 3.30 > Vcc 2.65 up to 3mA H Internal
Vcc = Vback
X1227