8
FN8085.3
July 29, 2005
the ISL1208 for up to 10 years. Another option is to use a
Super Cap for applications where V
DD
is interrupted for up
to a month. See the Applications Section for more
information.
Normal Mode (V
DD
) to Battery Backup Mode
(V
BAT
)
To transition from the V
DD
to V
BAT
mode, both of the
following conditions must be met:
Condition 1:
V
DD
< V
BAT
- V
BATHYS
where V
BATHYS
≈
50mV
Condition 2:
V
DD
< V
TRIP
where V
TRIP
≈
2.2V
Battery Backup Mode (V
BAT
) to Normal Mode
(V
DD
)
The ISL1208 device will switch from the V
BAT
to V
DD
mode
when one of the following conditions occurs:
Condition 1:
V
DD
> V
BAT
+ V
BATHYS
where V
BATHYS
≈
50mV
Condition 2:
V
DD
> V
TRIP
+ V
TRIPHYS
where V
TRIPHYS
≈
30mV
These power control situations are illustrated in Figures 9
and 10.
The I
2
C bus is deactivated in battery backup mode to provide
lower power. Aside from this, all RTC functions are
operational during battery backup mode. Except for SCL and
SDA, all the inputs and outputs of the ISL1208 are active
during battery backup mode unless disabled via the control
register. The User SRAM is operational in battery backup
mode down to 2V.
Power Failure Detection
The ISL1208 provides a Real Time Clock Failure Bit (RTCF)
to detect total power failure. It allows users to determine if
the device has powered up after having lost all power to the
device (both V
DD
and V
BAT
).
Low Power Mode
The normal power switching of the ISL1208 is designed to
switch into battery backup mode only if the V
DD
power is
lost. This will ensure that the device can accept a wide range
of backup voltages from many types of sources while reliably
switching into backup mode. Another mode, called Low
Power Mode, is available to allow direct switching from V
DD
to V
BAT
without requiring V
DD
to drop below V
TRIP
. Since
the additional monitoring of V
DD
vs V
TRIP
is no longer
needed, that circuitry is shut down and less power is used
while operating from V
DD
. Power savings are typically
600nA at V
DD
= 5V. Low Power Mode is activated via the
LPMODE bit in the control and status registers.
Low Power Mode is useful in systems where V
DD
is normally
higher than V
BAT
at all times. The device will switch from
V
DD
to V
BAT
when V
DD
drops below V
BAT
, with about 50mV
of hysteresis to prevent any switchback of V
DD
after
switchover. In a system with a V
DD
= 5V and backup lithium
battery of V
BAT
= 3V, Low Power Mode can be used.
However, it is not recommended to use Low Power Mode in
a system with V
DD
= 3.3V ±10%, V
BAT
≥
3.0V, and when
there is a finite I-R voltage drop in the V
DD
line.
InterSeal™ Battery Saver
The ISL1208 has the InterSeal™ Battery Saver which
prevents initial battery current drain before it is first used. For
example, battery-backed RTCs are commonly packaged on
a board with a battery connected. In order to preserve
battery life, the ISL1208 will not draw any power from the
battery source until after the device is first powered up from
the V
DD
source. Thereafter, the device will switchover to
battery backup mode whenever V
DD
power is lost.
Real Time Clock Operation
The Real Time Clock (RTC) uses an external 32.768kHz
quartz crystal to maintain an accurate internal representation
of second, minute, hour, day of week, date, month, and year.
The RTC also has leap-year correction. The clock also
corrects for months having fewer than 31 days and has a bit
that controls 24 hour or AM/PM format. When the ISL1208
powers up after the loss of both V
DD
and V
BAT
, the clock will
V
BAT
- V
BATHYS
V
BAT
V
BAT
+ V
BATHYS
BATTERY BACKUP
MODE
V
DD
V
TRIP
2.2V
1.8V
FIGURE 9. BATTERY SWITCHOVER WHEN V
BAT
< V
TRIP
FIGURE 10. BATTERY SWITCHOVER WHEN V
BAT
> V
TRIP
V
TRIP
V
BAT
V
TRIP
+ V
TRIPHYS
BATTERY BACKUP
MODE
V
DD
V
TRIP
3.0V
2.2V
ISL1208
