User's Manual

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08 Objective Specification Design-in
Page 83 of 188
Keep in mind that the use of rechargeable batteries requires the implementation of a suitable charger circuit
which is not included in SARA-G3 and SARA-U2 series modules. The charger circuit has to be designed to
prevent over-voltage on VCC pins of the module, and it should be selected according to the application
requirements: a DC/DC switching charger is the typical choice when the charging source has an high nominal
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. Refer to sections 2.2.1.7, 2.2.1.8
2.2.1.6, 2.2.1.9, and 2.2.1.10 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 withstand the maximum peak 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 withstand the maximum peak current of the module,
it must be able to withstand at least the maximum average 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 7.
For the additional specific guidelines for SARA-G350 ATEX modules integration in potentially explosive
atmospheres applications, refer to 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 7:
 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 (refer to the SARA-G3 series Data Sheet [1]
or the SARA-U2 series Data Sheet [2])
 Low output ripple: the switching regulator together with its output circuit must be capable of providing a
clean (low noise) VCC voltage profile
 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