ISL12008 Datasheet, PDF (7/19 Pages) Intersil Corporation – I2C Real Time Clock with Battery Backup

ISL12008
the crystal timing accuracy over-temperature either during
manufacturing or with an external temperature sensor and
microcontroller for active compensation (see Figure 6).
X1
X2
FIGURE 6. RECOMMENDED CRYSTAL CONNECTION
V_BAT
This input provides a backup supply voltage to the device.
V_BAT supplies power to the device in the event that the V_DD
supply fails. This pin can be connected to a battery, a super
capacitor or tied to ground if not used.
FT/OUT (512Hz Frequency Output/Logic Output)
This dual function pin can be used as a 512Hz frequency
output pin for on-chip crystal compensation or a simple
digital output control via I²C. The FT/OUT mode is selected
via the OUT and FT control bits of the control/status register
(address 07h). This pin is an open drain output requires the
use of a pull-up resistor.
Serial Clock (SCL)
The SCL input is used to clock all serial data into and out of
the device. The input buffer on this pin is always active (not
gated). It is disabled when the backup power supply on the
V_BAT pin is activated to minimize power consumption.
Serial Data (SDA)
SDA is a bidirectional pin used to transfer data into and out
of the device. It has an open drain output and may be ORed
with other open drain or open collector outputs. The input
buffer is always active (not gated) in normal mode.
An open drain output requires the use of a pull-up resistor.
The output circuitry controls the fall time of the output signal
with the use of a slope controlled pull-down. The circuit is
designed for 400kHz I²C bus speeds. It is disabled when the
backup power supply on the VBAT pin is activated.
V_DD, GND
Chip power supply and ground pins. The device will operate
with a power supply from 2.7V to 5.5VDC. A 0.1µF
decoupling capacitor is recommended on the V_DD pin to
ground.
Functional Description
Power Control Operation
The power control circuit accepts a V_DD and a V_BAT input.
Many types of batteries can be used with Intersil RTC
products. For example, 3.0V or 3.6V Lithium batteries are
appropriate, and battery sizes are available that can power
the ISL12008 for up to 10 years. Another option is to use a
super capacitor for applications where V_DD is interrupted for
up to a month. See “Application Section” on page 16 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 ≈ 2.6V
Battery Backup Mode (V_BAT) to Normal Mode
(V_DD)
The ISL12008 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 7
and 8.
V_DD
V_TRIP
V_BAT
V_BAT - V_BATHYS
BATTERY BACKUP
MODE
2.6V
1.8V
V_BAT + V_BATHYS
FIGURE 7. BATTERY SWITCHOVER WHEN V_BAT < V_TRIP
V_DD
V_BAT
V_TRIP
V_TRIP
BATTERY BACKUP
MODE
3.0V
2.6V
V_TRIP + V_TRIPHYS
FIGURE 8. BATTERY SWITCHOVER WHEN V_BAT > V_TRIP
7
FN6690.1
September 26, 2008