u-blox SARA-G3 Series System Integration Manual page 93

directly connected to VCC is the usual choice for battery-powered devices. During charging, batteries with
Ni-MH chemistry typically reach a maximum voltage that is above the maximum rating for VCC, and
should therefore be avoided.
The use of a primary (not rechargeable) battery is uncommon, since the most cells available are seldom
capable of delivering the burst peak current for a GSM call due to high internal resistance.
Keep in mind that the use of batteries requires the implementation of a suitable charger circuit (not
included in SARA-G3 modules). The charger circuit should be designed in order to prevent over-voltage
on VCC beyond the upper limit of the absolute maximum rating.
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
SARA-G3 series Data Sheet
specified in the
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
Sheet
[1]. The additional energy required by the module during a GSM/GPRS Tx slot (when in the worst
case the current consumption can rise up to 1.9 A, as described in section 1.5.1.2) 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 mΩ. 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
For the additional specific guidelines for SARA-G350 ATEX modules integration in potentially
explosive atmospheres applications, refer to section 2.13.
2.1.1.2 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
 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 1.9 A
current pulses with 1/8 duty cycle to the VCC pins
 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
UBX-13000995 - R06
SARA-G3 series - System Integration Manual
[1] is generally not recommended. However, if the selected
Table 6Table 6:
Objective Specification
SARA-G3 series Data
Table 6Table 6.
Design-in
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