24
FN8232.8
August 12, 2010
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 V
DD
to charge the
supercapacitor, which is connected to the V
BAT
pin. Try to
use Schottky diodes with very low leakages, <1µA
desirable.
Do not use the diode to charge a battery (especially lithium
batteries!).
Note that whether a battery or supercap is used, if the V
BAT
voltage drops below the data sheet minimum of 1.8V and the
V
DD
power cycles to 0V then back to V
DD
voltage, then the
RESET
output may stay low and the I
2
C communications will
not operate. The V
BAT
and V
DD
power will need to be cycled
to 0V together to allow normal operation again.
There are two possible modes for battery backup operation,
Standard and Legacy mode. In Standard mode, there are no
operational concerns when switching over to battery backup
since all other devices functions are disabled. Battery drain
is minimal in Standard mode, and return to Normal V
DD
powered operations predictable. In Legacy modes the V
BAT
pin can power the chip if the voltage is above V
DD
and
V
TRIP
.
This makes it possible to generate alarms and
communicate with the device under battery backup, but the
supply current drain is much higher than the Standard mode
and backup time is reduced. During initial power-up, the
default mode is the Legacy mode.
I
2
C Communications During Battery Backup and
LVR Operation
Operation in Battery Backup mode and LVR is affected by
the BSW and SBIB bits as described earlier. These bits allow
flexible operation of the serial bus and EEPROM in battery
backup mode, but certain operational details need to be
clear before utilizing the different modes. The most
significant detail is that once V
DD
goes below V
RESET
, then
I
2
C communications cease regardless of whether the device
is programmed for I
2
C operation in battery backup mode.
Table 10 describes 4 different modes possible with using the
BSW and SBIB bits, and how they are affect LVR and battery
backup operation.
• Mode A - In this mode, selection bits indicate a low V
DD
switchover combined with I
2
C operation in battery backup
mode. In actuality the V
DD
will go below V
RESET
before
switching to battery backup, which will disable I
2
C
ANYTIME the device goes into battery backup mode.
Regardless of the battery voltage, the I
2
C will work down
to the V
RESET
voltage (see Figure 29).
• Mode B - In this mode, the selection bits indicate
switchover to battery backup at V
DD
<V
BAT
, and I
2
C
communications in battery backup. In order to
communicate in battery backup mode, the V
RESET
voltage
must be less than the V
BAT
voltage AND V
DD
must be
greater than V
RESET
. Also, pull-ups on the I
2
C bus pins
must go to V
BAT
to communicate. This mode is the same
as the normal operating mode of the X1228 device.
• Mode C - In this mode, the selection bits indicate a low
V
DD
switchover combined with no communications in
battery backup. Operation is actually identical to Mode A
with I
2
C communications down to V
DD
= V
RESET
, then no
communications (see Figure 28).
• Mode D - In this mode, the selection bits indicate
switchover to battery backup at V
DD
< V
BAT
, and no I
2
C
communications in battery backup. This mode is unique in
that there is I
2
C communication as long as V
DD
is higher
than V
RESET
or V
BAT
, whichever is greater. This mode is
the safest for guaranteeing I
2
C communications only when
there is a Valid V
DD
(see Figure 29).
FIGURE 28. SUPERCAPACITOR CHARGING CIRCUIT
V
DD
V
BAT
V
SS
SUPERCAPACITOR
2.7V TO 5.5V
ISL12027, ISL12027A