SARA-N2 series - System Integration Manual
UBX-17005143 - R06
Design-in
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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 simultaneously available in the
application as possible supply sources, 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.
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 be mutually exclusive.
The usage of supercapacitors on the VCC supply line is generally not recommended since these
components are highly temperature sensitive and may increase current leakages draining the battery
faster.
The following sections highlight some design aspects for power-supply scenarios, providing
application circuit design-in compliant with the module VCC requirements summarized in
☞
For the additional specific requirements applicable to the integration of SARA-N211 modules in
devices intended for use in potentially explosive atmospheres, see section
2.2.1.2
Guidelines to optimize power consumption
The NB-IoT technology is primarly intended for applications that require small amount of data
exchange per day (i.e. few bytes in uplink and downlink per day) and these are typically battery
powered. Depending on the application type, an operating life of 5 to 15 years is usually required. For
these reasons, the whole application board should be optimized in terms of current consumption and
should carefully take into account the following aspects:
Minimize current leakages on the power supply line
Optimize the antenna matching since an un-matched antenna leads to higher current
consumptions
Use an application processor with UART interface working at the same level of the VCC supply
input of the SARA-N2 module (for example, 3.3 V or 3.6 V). In this way it is possible to avoid voltage
translators on the UART interface, which operates at the VCC voltage level
The application processor should go in standby (or lowest power consumption mode) as soon as
the SARA-N2 module enters the deep-sleep mode and there’s no more data to be transmitted: the
module will automatically enter the deep-sleep mode whenever possible to limit current
consumption and avoid further network registration procedures each time there is an up-link
message to be transmitted.
The application processor can monitor the V_INT level to sense when radio is on or off.
The application processor can detect the presence of down-link messages monitoring the CTS
pin, which provides the Ring Indicator functionality, notifying incoming data received by the
module or an URC event.
Possibility to request new network timers and select the optimum set of values depending on the
intended application use case
2.2.1.3
Guidelines for VCC supply circuit design using a primary battery
The characteristics of a battery connected to VCC pins should meet the following prerequisites to
comply with the module VCC requirements summarized in
Maximum pulse and DC discharge current: the non-rechargeable battery with its output circuit
must be capable of delivering to VCC pins the specified average current during a transmission at
