User's Manual
SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R18 Advance Information Design-in
Page 92 of 206
voltage (e.g. ~12 V), whereas a linear charger is the typical choice when the charging source has a relatively low
nominal voltage (~5 V). If both a permanent primary supply / charging source (e.g. ~12 V) and a rechargeable
back-up battery (e.g. 3.7 V Li-Pol) are available at the same time in the application as possible supply source,
then a proper charger / regulator with integrated power path management function can be selected to supply
the module while simultaneously and independently charging the battery. See sections 2.2.1.8, 2.2.1.9, 2.2.1.6,
2.2.1.10, and 2.2.1.11 for specific design-in.
The use of a primary (not rechargeable) battery is in general uncommon, but appropriate parts can be selected
given that the most cells available are seldom capable of delivering the burst peak current for a GSM call due to
high internal resistance. See sections 2.2.1.5, 2.2.1.6, 2.2.1.10, and 2.2.1.11 for specific design-in.
The usage of more than one DC supply at the same time should be carefully evaluated: depending on the supply
source characteristics, different DC supply systems can result as mutually exclusive.
The usage of a regulator or a battery not able to support the highest peak of VCC current consumption specified
in the SARA-G3 series Data Sheet [1] and in the SARA-U2 series Data Sheet [2] is generally not recommended.
However, if the selected regulator or battery is not able to support the highest peak current of the module, it
must be able to support at least the highest averaged current consumption value specified in the SARA-G3 series
Data Sheet [1] and in the SARA-U2 series Data Sheet [2]. The additional energy required by the module during a
2G Tx slot can be provided by an appropriate bypass tank capacitor or supercapacitor with very large capacitance
and very low ESR placed close to the module VCC pins. Depending on the actual capability of the selected
regulator or battery, the required capacitance can be considerably larger than 1 mF and the required ESR can be
in the range of few tens of m:. Carefully evaluate the implementation of this solution since aging and
temperature conditions significantly affect the actual capacitor characteristics.
The following sections highlight some design aspects for each of the supplies listed above providing application
circuit design-in compliant with the module VCC requirements summarized in Table 6.
For the additional specific guidelines for SARA-G340 ATEX, SARA-G350 ATEX, SARA-U201 ATEX and
SARA-U270 ATEX modules integration in potentially explosive atmospheres applications, see the section
2.14.
2.2.1.2 Guidelines for VCC supply circuit design using a switching regulator
The use of a switching regulator is suggested when the difference from the available supply rail to the VCC value
is high: switching regulators provide good efficiency transforming a 12 V or greater voltage supply to the typical
3.8 V value of the VCC supply.
The characteristics of the switching regulator connected to VCC pins should meet the following prerequisites to
comply with the module VCC requirements summarized in Table 6:
x Power capability: the switching regulator with its output circuit must be capable of providing a voltage
value to the VCC pins within the specified operating range and must be capable of delivering to VCC pins
the specified maximum peak / pulse current with 1/8 duty cycle (see the SARA-G3 series Data Sheet [1] or
the SARA-U2 series Data Sheet [2]).
x Low output ripple: the switching regulator together with its output circuit must be capable of providing a
clean (low noise) VCC voltage profile.
x High switching frequency: for best performance and for smaller applications select a switching frequency
≥ 600 kHz (since L-C output filter is typically smaller for high switching frequency). The use of a switching
regulator with a variable switching frequency or with a switching frequency lower than 600 kHz must be
carefully evaluated since this can produce noise in the VCC voltage profile and therefore negatively impact
GSM modulation spectrum performance. An additional L-C low-pass filter between the switching regulator
output to VCC supply pins can mitigate the ripple on VCC, but adds extra voltage drop due to resistive
losses on series inductors.