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ISL1208IU8Z View Datasheet(PDF) - Intersil

Part NameDescriptionManufacturer
ISL1208IU8Z I2C® Real Time Clock/Calendar Intersil
Intersil Intersil
ISL1208IU8Z Datasheet PDF : 24 Pages
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ISL1208
the ISL1208 for up to 10 years. Another option is to use a
Super Cap for applications where VDD is interrupted for up
to a month. See the “Application Section” on page 18 for
more information.
Normal Mode (VDD) to Battery Backup Mode
(VBAT)
To transition from the VDD to VBAT mode, both of the
following conditions must be met:
Condition 1:
VDD < VBAT - VBATHYS
where VBATHYS 50mV
Condition 2:
VDD < VTRIP
where VTRIP 2.2V
Battery Backup Mode (VBAT) to Normal Mode
(VDD)
The ISL1208 device will switch from the VBAT to VDD mode
when one of the following conditions occurs:
Condition 1:
VDD > VBAT + VBATHYS
where VBATHYS 50mV
Condition 2:
VDD > VTRIP + VTRIPHYS
where VTRIPHYS 30mV
These power control situations are illustrated in Figures 9
and 10.
VDD
VTRIP
VBAT
VBAT - VBATHYS
BATTERY BACKUP
MODE
2.2V
1.8V
VBAT + VBATHYS
FIGURE 9. BATTERY SWITCHOVER WHEN VBAT < VTRIP
VDD
VBAT
VTRIP
VTRIP
BATTERY BACKUP
MODE
3.0V
2.2V
VTRIP + VTRIPHYS
FIGURE 10. BATTERY SWITCHOVER WHEN VBAT > VTRIP
The I2C 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 VDD and VBAT).
Low Power Mode
The normal power switching of the ISL1208 is designed to
switch into battery backup mode only if the VDD 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 VDD
to VBAT without requiring VDD to drop below VTRIP. Since
the additional monitoring of VDD vs VTRIP is no longer
needed, that circuitry is shut down and less power is used
while operating from VDD. Power savings are typically
600nA at VDD = 5V. Low Power Mode is activated via the
LPMODE bit in the control and status registers.
Low Power Mode is useful in systems where VDD is normally
higher than VBAT at all times. The device will switch from
VDD to VBAT when VDD drops below VBAT, with about 50mV
of hysteresis to prevent any switchback of VDD after
switchover. In a system with a VDD = 5V and backup lithium
battery of VBAT = 3V, Low Power Mode can be used.
However, it is not recommended to use Low Power Mode in
a system with VDD = 3.3V ±10%, VBAT 3.0V, and when
there is a finite I-R voltage drop in the VDD 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 VDD source. Thereafter, the device will switchover to
battery backup mode whenever VDD 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 VDD and VBAT, the clock will not begin
incrementing until at least one byte is written to the clock
register.
9
FN8085.8
September 12, 2008
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