The Smart Timing Choice
The Smart Timing Choice
SiT1552
Ultra-Low Power, Ultra-Small 32.768 kHz MEMS TCXO
Rev. 0.85 Page 4 of 9 www.sitime.com
Description
The SiT1552 is an ultra-small and ultra-low power 32.768 kHz
TCXO optimized for battery-powered applications. SiTime’s
silicon MEMS technology enables the first 32 kHz TCXO in
the world’s smallest footprint and chip-scale packaging
(CSP). Typical core supply current is only 1 µA. And unlike
standard oscillators, the SiT1552 features NanoDrive™, a
factory programmable output that reduces the voltage swing
to minimize power.
SiTime’s MEMS oscillators consist of MEMS resonators and
a programmable analog circuit. Our MEMS resonators are
built with SiTime’s unique MEMS First™ process. A key
manufacturing step is EpiSeal™ during which the MEMS
resonator is annealed with temperatures over 1000°C.
EpiSeal creates an extremely strong, clean, vacuum chamber
that encapsulates the MEMS resonator and ensures the best
performance and reliability. During EpiSeal, a poly silicon cap
is grown on top of the resonator cavity, which eliminates the
need for additional cap wafers or other exotic packaging. As
a result, SiTime’s MEMS resonator die can be used like any
other semiconductor die. One unique result of SiTime’s
MEMS First and EpiSeal manufacturing processes is the
capability to integrate SiTime’s MEMS die with a SOC, ASIC,
microprocessor or analog die within a package to eliminate
external timing components and provide a highly integrated,
smaller, cheaper solution to the customer.
TCXO Frequency Stability
The SiT1552 is factory calibrated (trimmed) over multiple
frequency points to guarantee extremely tight stability over
temperature. Unlike quartz crystals that have a classic tuning
fork parabola temperature curve with a 25°C turnover point
with a 0.04 ppm/C2 temperature coefficient, the SiT1552
temperature coefficient is calibrated and corrected over
temperature with an active temperature correction circuit. The
result is 32 kHz TCXO with extremely tight frequency variation
over the -40°C to +85°C temperature range. Contact SiTime
for applications that require a wider supply voltage range
>3.63V, or lower operating frequency below 32 kHz.
When measuring the SiT1552 output frequency with a
frequency counter, it is important to make sure the counter's
gate time is >100ms. The slow frequency of a 32kHz clock will
give false readings with faster gate times.
Power Supply Noise Immunity
In addition to eliminating external output load capacitors
common with standard XTALs, this device includes special
power supply filtering and thus, eliminates the need for an
external Vdd bypass-decoupling capacitor to keep the
footprint as small as possible. Internal power supply filtering
is designed to reject more than 500 mV noise and frequency
components from low frequency to more than 10 MHz.
Start-up and Steady-State Supply Current
The SiT1552 TCXO starts-up to a valid output frequency
within 300 ms (150ms typ). To ensure proper start-up, Vdd
power-supply ramp, from a power-down state to 90% of final
Vdd, must be less than 100ms.
During initial power-up, the SiT1552 power-cycles internal
blocks, as shown in the power-supply start-up and steady
state plot in the Typical Operating Curves section. Power-up
and initialization is typically 200 ms, and during that time, the
peak supply current reaches 28 µA as the internal capacitors
are charged, then sequentially drops to its 990 nA
steady-state current. During steady-state operation, the
internal temperature compensation circuit turns on every 350
ms for a duration of approximately 10 ms.
Output Voltage
The SiT1552 has two output voltage options. One option is a
standard LVCMOS output swing. The second option is the
NanoDrive reduced swing output. Output swing is customer
specific and Factory programmed between 200 mV and 800
mV. For DC-coupled applications, output VOH and VOL are
individually factory programmed to the customers’
requirement. VOH programming range is between 600 mV
and 1.225V in 100 mV increments. Similarly, VOL
programming range is between 350 mV and 800 mV. For
example; a PMIC or MCU is internally 1.8V logic compatible,
and requires a 1.2V VIH and a 0.6V VIL. Simply select SiT1552
NanoDrive factory programming code to be “D14” and the
correct output thresholds will match the downstream PMIC or
MCU input requirements. Interface logic will vary by manufac-
turer and we recommend that you review the input voltage
requirements for the input interface.
For DC-biased NanoDrive output configuration, the minimum
VOL is limited to 350mV and the maximum allowable swing
(VOH - VOL) is 750mV. For example, 1.1V VOH and 400mV
VOL is acceptable, but 1.2V VOH and 400 mV VOL is not
acceptable.
When the output is interfacing to an XTAL input that is inter-
nally AC-coupled, the SiT1552 output can be Factory
programmed to match the input swing requirements. For
example, if a PMIC or MCU input is internally AC-coupled and
requires an 800mV swing, then simply choose the SiT1552
NanoDrive programming code “AA8” in the part number. It is
important to note that the SiT1552 does not include internal
AC-coupling capacitors. Please see the Part Number
Ordering section at the end of the datasheet for more infor-
mation about the part number ordering scheme.