ISL12022 Datasheet, PDF (25/28 Pages) Intersil Corporation – Real Time Clock with On Chip ±5ppm Temp Compensation

ISL12022
Application Section
Battery-Backup Details
The ISL12022 has automatic switchover to battery-backup
when the V_DD drops below the VBAT mode threshold. A
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_BAT is 1.8V to 5.5V, but keep in
mind the temperature compensation only operates for V_BAT
> 2.7V. Note that the device is not guaranteed to operate
with a V_BAT < 1.8V, so the battery should be changed before
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_DD
and V_BAT are powered down together. To avoid that situation,
including situations where a battery may discharge deeply, the
circuit in Figure 18 can be used.
V_DD = 2.7V
TO 5.5V
C_IN
0.1µF
ISL12022
VDD VBAT
GND
J_BAT
D_BAT
BAT43W
C_BAT
0.1µF
+ _{VBAT = 1.8V}
TO 3.2V
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
in the vicinity of the crystal. These logic level lines can
induce noise in the oscillator circuit, causing misclocking.
• Add a ground trace around the crystal with one end
terminated at the chip ground. This will provide termination
for emitted noise in the vicinity of the RTC device.
FIGURE 18. SUGGESTED BATTERY-BACKUP CIRCUIT
The diode, D_BAT will add a small drop to the battery voltage
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_DD negative slew rate should be limited to below the
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_DD and the I²C pull-ups. This is not advised, as it may
compromise the operation of the I²C bus. For applications
that do require serial bus communication with the RTC V_DD
powered down, the SDA pin must be pulled low during the
time the RTC V_DD ramps down to 0V. Otherwise, the device
may lose serial bus communications once V_DD is powered
up, and will return to normal operation ONLY once V_DD and
V_BAT are both powered down together.
Oscillator Crystal Requirements
The ISL12022 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 26
25
FIGURE 19. SUGGESTED LAYOUT FOR ISL12022 AND
CRYSTAL
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_OUT pin is used as a clock, it should be
routed away from the RTC device as well. The traces for the
V_BAT and V_DD pins can be treated as a ground, and should
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
FN6659.2
June 23, 2009