LENA-R8 series - System integration manual
UBX-22015376 - R02
Design-in
Page 34 of 116
C1-Public
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 a 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
for specific design-in.
An appropriate primary (not rechargeable) battery can be selected taking into account the maximum
current specified in the LENA-R8 series data sheet
during connected mode, considering that
primary cells might have weak power capability. See sections
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 a regulator or a battery not able to support the highest peak of
VCC
current consumption
specified in the LENA-R8 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 at least the highest averaged current consumption value specified in the LENA-R8 series
data sheet
with an adequate margin. The additional energy required by the module during a 2G Tx
slot can be provided by an appropriate bypass tank capacitor or 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 DC/DC 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 the
VCC
pins should meet the following
prerequisites to comp
ly with the module’s
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 the
VCC
pins the specified maximum peak / pulse current consumption during Tx
burst at the maximum Tx power specified in the LENA-R8 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 the L-C output filter is typically smaller for high
switching frequencies). The use of a switching regulator with a variable switching frequency or
with a switching frequency lower than 600 kHz must be evaluated carefully, since this can produce
noise in the
VCC
voltage profile and therefore negatively impact modulation spectrum
performance.