ISL12022 INTERSIL [Intersil Corporation], ISL12022 Datasheet - Page 25

• Manufacturer Part Number: ISL12022
• Manufacturer: INTERSIL [Intersil Corporation]

Application Section
Battery-Backup Details
The ISL12022 has automatic switchover to battery-backup
when the V
wide variety of backup sources can be used, including
standard and rechargeable lithium, super capacitors, or
regulated secondary sources. The serial interface is
disabled in battery-backup, while the oscillator and RTC
registers are operational. The SRAM register contents are
powered to preserve their contents as well.
The input voltage range for V
mind the temperature compensation only operates for V
> 2.7V. Note that the device is not guaranteed to operate
with a V
discharging to that level. It is strongly advised to monitor the
low battery indicators in the status registers and take action
to replace discharged batteries.
If a supercapacitor is used, it is possible that it may discharge to
below 1.8V during prolonged power-down. Once powered up,
the device may lose serial bus communications until both V
and V
including situations where a battery may discharge deeply, the
circuit in Figure 18 can be used.
The diode, D
but will protect the circuit should battery voltage drop below
1.8V. The jumper is added as a safeguard should the battery
ever need to be disconnect from the circuit.
The V
data sheet spec (10V/ms) otherwise battery switchover can
be delayed, resulting in SRAM contents corruption and
oscillator operation interruption.
Some applications will require separate supplies for the RTC
V
compromise the operation of the I
that do require serial bus communication with the RTC V
powered down, the SDA pin must be pulled low during the
time the RTC V
may lose serial bus communications once V
up, and will return to normal operation ONLY once V
V
Oscillator Crystal Requirements
The ISL12022 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 26
V
DD
BAT
0.1µF
DD
TO 5.5V
C
FIGURE 18. SUGGESTED BATTERY-BACKUP CIRCUIT
IN
= 2.7V
and the I
BAT
are both powered down together.
DD
BAT
negative slew rate should be limited to below the
are powered down together. To avoid that situation,
DD
< 1.8V, so the battery should be changed before
BAT
VDD
2
drops below the VBAT mode threshold. A
C pull-ups. This is not advised, as it may
DD
ISL12022
will add a small drop to the battery voltage
GND
ramps down to 0V. Otherwise, the device
VBAT
25
BAT
J
BAT
is 1.8V to 5.5V, but keep in
2
C bus. For applications
BAT43W
C
0.1µF
D
BAT
BAT
DD
is powered
+
V
BAT
TO 3.2V
DD
= 1.8V
BAT
and
DD
DD
ISL12022
lists some recommended surface mount crystals and the
parameters of each. This list is not exhaustive and other
surface mount devices can be used with the ISL12022 if their
specifications are very similar to the devices listed. The crystal
should have a required parallel load capacitance of 12.5pF and
an equivalent series resistance of less than 50k. The crystal’s
temperature range specification should match the application.
Many crystals are rated for -10°C to +60°C (especially
through-hole and tuning fork types), so an appropriate crystal
should be selected if extended temperature range is required.
Layout Considerations
The crystal input at X1 has a very high impedance, and
oscillator circuits operating at low frequencies (such as
32.768kHz) are known to pick up noise very easily if layout
precautions are not followed. Most instances of erratic clocking
or large accuracy errors can be traced to the susceptibility of
the oscillator circuit to interference from adjacent high speed
clock or data lines. Careful layout of the RTC circuit will avoid
noise pickup and insure accurate clocking.
Figure 19 shows a suggested layout for the ISL12022 device
using a surface mount crystal. Two main precautions should
be followed:
• Do not run the serial bus lines or any high speed logic lines
• Add a ground trace around the crystal with one end
In addition, it is a good idea to avoid a ground plane under
the X1 and X2 pins and the crystal, as this will affect the load
capacitance and therefore the oscillator accuracy of the
circuit. If the ~IRQ/F
routed away from the RTC device as well. The traces for the
V
be routed around the crystal.
Applications Information
Crystal Oscillator Frequency Compensation
CRYSTAL CHARACTERISTICS
The ISL12022 device contains a complete system for
adjusting the frequency of the crystal oscillator to
BAT
FIGURE 19. SUGGESTED LAYOUT FOR ISL12022 AND
in the vicinity of the crystal. These logic level lines can
induce noise in the oscillator circuit, causing misclocking.
terminated at the chip ground. This will provide termination
for emitted noise in the vicinity of the RTC device.
and V
DD
CRYSTAL
pins can be treated as a ground, and should
OUT
pin is used as a clock, it should be
June 23, 2009
FN6659.2