TOBY-R2 series - System integration manual
UBX-16010572 - R10
Design-in
Page 66 of 151
Keep in mind that the use of rechargeable batteries requires the implementation of a suitable charger
circuit which is not included in the modules. The charger circuit has to be designed to prevent
over-voltage on VCC pins, 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 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
An appropriate primary (not rechargeable) battery can be selected taking into account the maximum
current specified in TOBY-R2 series data sheet
during connected-mode, considering that primary
cells might have weak power capability. See sections
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 TOBY-R2 series data sheet
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 with adequate margin at least the highest averaged current consumption value specified in
the TOBY-R2 series data sheet
. The additional energy required by the module during a 2G Tx slot
can be provided by an appropriate bypass tank capacitor or a super-capacitor 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 super-capacitor
characteristics since aging and temperature may affect the actual 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
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 source
to the VCC value is high, since 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 to VCC pins the maximum peak / pulse current consumption during Tx burst at
maximum Tx power specified in the TOBY-R2 series data sheet
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 it is recommended
to 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 LTE/3G/2G modulation spectrum
performance.
