ISL12023 Datasheet, PDF (25/28 Pages) Intersil Corporation – Low Power RTC with Battery-Backed SRAM and Embedded Temp Compensation ±5ppm with Auto Daylight Saving

ISL12023
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.
V_DD = 2.7V
TO 5.5V
C_IN
0.1µF
ISL12023
VDD VBAT
J_BAT
D_BAT
BAT43W
C_BAT
0.1µF
+ _{VBAT = 1.8V}
TO 3.2V
GND
FIGURE 19. 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.
Oscillator Crystal Requirements
The ISL12023 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 26
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 ISL12023 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.
TABLE 26. SUGGESTED SURFACE MOUNT CRYSTALS
MANUFACTURER
PART NUMBER
Citizen
Epson
CM200S
MC-405, MC-406
Raltron
RSM-200S
SaRonix
32S12
Ecliptek
ECPSM29T-32.768K
ECS
ECX-306
Fox
FSM-327
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 20 shows a suggested layout for the ISL12023 device
using a surface mount crystal. Two main precautions should
be followed:
1. 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 to cause misclocking.
2. 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 20. SUGGESTED LAYOUT FOR ISL12023 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 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.
Crystal Oscillator Frequency Compensation
CRYSTAL CHARACTERISTICS
The ISL12023 device contains a complete system for
adjusting the frequency of the crystal oscillator to
compensate for temperature variation. A typical 32.768kHz
crystal used with RTC devices has a temperature versus
frequency curve, as shown in Figure 21.
0
-20
-40
-60
-80
-100
-120
-140
-160
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80
TEMPERATURE (°C)
FIGURE 21. RTC CRYSTAL TEMPERATURE DRIFT
This curve equation follows Equation 6:
Δf = α • (T – T₀ )²
(EQ. 6)
25
FN6682.2
June 24, 2009