AN1744: ISL12022M Oscillator Accuracy

Application Note 1744
ISL12022M Oscillator Accuracy
Introduction
Datasheet Specifications
The ISL12022M devices are high-accuracy, temperature
compensated Real Time Clock (RTC) modules that incorporate
the RTC die and the quartz crystal. The RTC contains an
internal temperature sensor and state machine to adjust the
oscillator frequency and allow factory calibration to less than
±5ppm error over the -40°C to +85°C ambient operating
temperature range. There are numerous items that affect the
final accuracy of the module once assembled on a PC board,
including handling and shock, reflow temperature profile,
maximum temperature, number of reflows (or wave solder)
and if ultrasound is used (note that the ISL12022M family
refers to the ISL12022M, ISL12022MA and
ISL12022MR5421 products).
The datasheet provides separate accuracy specifications for
room temperature at a fixed VDD = 3.3V, over VDD supply
voltage variation, and over the rated temperature range. “DC
Operating Characteristics RTC” on page 2 shows a portion of
the datasheet extracted to show just the overall conditions and
the individual accuracy specs.
This application note will discuss the expected accuracy of the
device when mounted on a board, at room temperature and as
temperature varies. Also discussed will be the measurement
techniques and analyzing the results.
Expected Oscillator Accuracy and Time
Drift
The oscillator accuracy is summarized in Table 1. The main
point of this table is that after the module device is reflow
soldered to a PC board, the accuracy spec in the
ISL12022M datasheet will have additional errors such that the
resulting accuracy will exceed the maximum datasheet
specification. Also, due to the variability of the reflow process
and customer application PC boards, there cannot be a
maximum stated error after reflow, only the expected value
based on the estimated change. The best assurance of getting
the best accuracy is to minimize the peak time and
temperature of the assembly reflow process.
Let’s look more closely at each line item spec.
• “Oscillator Stability vs Temperature” refers to the maximum
variation from a perfect 0ppm error with the device at an
ambient temperature from -40°C to +85°C. Oscillator
stability in this case refers to the average frequency
measured over a period of time, since the digital correction
mechanism inserts periodic correction, at intervals of
greater than 1 second in most cases. A measurement at 1
or 2 second intervals may not provide an accurate or stable
reading due to the digital correction. The 5ppm is the
maximum guaranteed for a packaged device as it is shipped
to the customer.
• “Oscillator Stability vs Voltage” is the maximum deviation, at
room temperature, from a perfect 0ppm error as the VDD
voltage is varied from 2.7V to 5.5V.
• “Oscillator Initial Accuracy” is measured at TA = +25°C only;
the maximum variation from 0ppm error is at VDD = 3.3V,
VBAT = 0.0V.
• “Temperature Sensor Accuracy” is the maximum error at
TA = +25°C of the digital temperature reading (TEMP
registers) from exact chip temperature. It is listed here since
the oscillator calibration takes effect at <20°C and >30°C,
as measured by the chip as opposed to actual temperature.
Related Literature
• ISL12022M datasheet
• ISL12022MA datasheet
• ISL12022MR5421 datasheet
TABLE 1. ISL12022M OSCILLATOR ACCURACY SUMMARY
TEMP. RANGE >
+25°C
(ppm)
+25°C
(SEC/DAY)
-40°C to +85°C
(ppm)
-40°C to +85°C
(SEC/DAY)
Before Reflow
±3 (max)
0.26 (max)
±5 (max)
0.43 (max)
After Reflow
±6 (typically)
0.52 (typically)
±8 (typically)
0.69 (typically)
October 17, 2014
AN1744.0
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Application Note 1744
DC Operating Characteristics RTC Test Conditions: VDD = +2.7 to +5.5V, TA = -40°C to +85°C, unless otherwise stated. Boldface
limits apply across the operating temperature range, -40°C to +85°C.
SYMBOL
PARAMETER
CONDITIONS
MIN
(Note 1)
TYP
(Note 2)
MAX
(Note 1)
UNITS
OSCILLATOR ACCURACY
FoutI
Oscillator Initial Accuracy
VDD = 3.3V (Notes 3, 5)
FoutR
Oscillator Accuracy after Reflow Cycle
VDD  3.3V (Notes 3, 5)
FoutT
Oscillator Stability vs Temperature
VDD  3.3V (Notes 3, 6)
FoutV
Oscillator Stability vs Voltage
2.7V  VDD  5.5V (Note 7)
Temp
Temperature Sensor Accuracy
VDD = VBAT = 3.3V (Note 4)
-2
+8
±5
±2
-3
ppm
+3
±2
ppm
ppm
ppm
°C
NOTES:
1. Parameters with MIN and/or MAX limits are 100% tested at +25°C, unless otherwise specified. Temperature limits established by characterization
and are not production tested.
2. Specified at +25°C.
3. The ISL12022M Oscillator Initial Accuracy can change after solder reflow attachment. The amount of change will depend on the reflow temperature
and length of exposure. A general rule is to use only one reflow cycle and keep the temperature and time as short as possible. Changes on the order
of ±1ppm to ±3ppm can be expected with typical reflow profiles.
4. Limits should be considered typical and are not production tested.
5. Defined as the deviation from a target oscillator frequency of 32,768.0Hz at room temperature.
6. Defined as the deviation from the room temperature measured 1Hz frequency, VDD = 3.3V, at TA = -40°C to +85°C.
7. Defined as the deviation at room temperature from the measured 1Hz frequency (or equivalent) at VDD = 3.3, over the range of VDD = 2.7V to
VDD = 5.5V.
In addition to the error specs listed, Note 3 is pertaining to the
error that results from high temperature assembly reflow.
This note indicates that an additional tolerance of typically up to
3ppm is added to the error specification after reflow. Combining
this change with the initial accuracy specs gives the ±6ppm
initial accuracy expected value at +25°C, as shown in Table 1 on
page 1.
Environmental Effects
Thermal Effects
SOLDER REFLOW HEAT, PCB ASSEMBLY
Typical surface mount production assembly flows include at least
one infrared reflow pass for soldering components to pads on the
PC board. Heating profiles for reflow cycles have peak
temperatures of up to 260°C for 10 to 30 seconds, which is high
enough to slightly change the characteristic of the quartz crystal
in the RTC module. After cooling, the typical final effect on the
module is a slightly lower oscillator frequency (sometimes but
not often higher frequency). Therefore, the RTC module after
reflow assembly, will have a different accuracy range than that
specified on the datasheet as shipped from the factory. See
Table 1 on page 1.
The customer should take this accuracy change into account
when considering the final system accuracy for the assembled
RTC module on their PC board. Also, every effort needs to be
made to minimize the peak reflow temperature and peak time
as well and the number of reflow cycles. Thus, reducing the
frequency change after reflow. RTC module devices can have
accuracy error exceeding 8ppm and still be within the expected
range after a production reflow at +260°C peak temperature.
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Mechanical Effects
PCB CUTTING WITH ROUTERS
Usually at the end of the pick and place process, a PCB panel
needs to have the single PCBs separated from each other with a
cutting tool or router. These machines sometimes generate
vibrations on the PCB that have a fundamental or harmonic
frequencies close to 32.768kHz. This might cause breakage of
the crystal blanks inside the module due to resonance. Note: The
customer should monitor their cutting tool speed and the speed
should be adjusted to avoid resonant vibration. If this is not done,
there can be very large accuracy error in the RTC oscillator or
sometimes even failure of the oscillator if the crystal is damaged
sufficiently.
ULTRASOUND CLEANING
Ultrasound cleaning of PCBs is a common practice after
assembly. This cleaning removes any material from the soldering
process by submersing in a bath and using high frequency
ultrasound to loosen the foreign material.
The RTC crystal can be damaged by this process if a lower
frequency is used and the intensity is high enough to cause
resonance in the crystal. Generally, frequencies higher than
500kHz should avoid damage and also any harmonic of the
32.768kHz frequency should be avoided. Note: The customer
should monitor the ultrasound frequency and the frequency
should be adjusted to avoid crystal harmonics. Failure to observe
proper ultrasound frequency control can result in either large RTC
oscillator frequency error or possible oscillator failure due to a
damaged crystal.
AN1744.0
October 17, 2014
Application Note 1744
MECHANICAL SHOCK AND VIBRATION
Possible sources of crystal frequency offset include mechanical
shock. The datasheet lists a maximum shock resistance of
5000g for 0.3ms, 1/2 sine test, which can give a ±5ppm
frequency change. Likewise, the maximum vibration spec is 20g
at 10 to 2000Hz. Note that ultrasound cleaning and PCB panel
cutting vibrations fall under the broader mechanical shock and
vibration spec as well. The customer should avoid any
mechanical handling that produces an impact or high vibration
on the module. Failure to observe proper precautions handling
the device, before or after, assembly can result in higher than
expected oscillator frequency error.
Appendix
Figure 1 is sample data for the ISL12022M parts that were
subjected to 1 reflow cycle. The parts were socket tested and
frequency accuracy data recorded, then sent through a
production reflow oven and allowed to cool and settle. The parts
were then socket tested again and the data recorded, with the
change or delta frequency out calculated.
Parts soldered to a board may exhibit less change after reflow as
the heat is distributed across the PC board. As the devices are
cooled they maintain shape when attached to the PC board.
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Summary
For final product oscillator accuracy, the customer must consider
the assembly effects and the resulting board mounted module
device may not be within the expected accuracy limits shown on
the datasheet. The customer can adjust their system to address
this change in accuracy or look into other ways of adjusting final
oscillator accuracy.
20
15
10
4
3
2
0
1
2
3
4
5
6
7
8
0
9
5
10
# UNITS
The ISL12022M provides a high accuracy 3-in-1 RTC solution for
real time clock timing requirements. The oscillator accuracy is
guaranteed for the device shipped from the factory. The PCB
assembly process adds environmental factors, such as heat and
vibration that will cause the accuracy error to increase after
assembly. This change will be evident when testing the final
assembled PC board RTC oscillator frequency. The major
contributors to additional accuracy error are solder reflow, board
cutting or routing, ultrasound cleaning and mechanical handling
shocks or vibrations.
ppm CHANGE
FIGURE 1. SAMPLE REFLOW DATA
NOTE: If the customer has the capability to test, access the serial
interface, make adjustments and test again, there are methods to
readjust the device for better accuracy after assembly. Please contact
Intersil applications about this possibility.
The customer needs to read and understand the environmental
effects on the crystal oscillator accuracy and make every effort to
minimize these effects, which will result in the highest possible
accuracy of the board mounted RTC. In some cases, the
customer may be forced to use an assembly process that is
known to cause additional accuracy error as listed in this
document. They should immediately contact Intersil to verify the
hazard and possible impact and to identify alternative
procedures that may be possible.
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is
cautioned to verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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AN1744.0
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