SARA-R4 series - System integration manual
UBX-16029218 - R20 Design-in Page 43 of 128
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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 a 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 suitable charger / regulator with integrated power path management function can be
selected to supply the module while simultaneously and independently charging the battery. See
sections 2.2.1.6 and 2.2.1.7 for specific design-in.
An appropriate primary (not rechargeable) battery can be selected taking into account the maximum
current specified in the SARA-R4 series data sheet [1] during connected mode, considering that
primary cells might have weak power capability. See section 2.2.1.5 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 selected regulator or battery must be able to support with adequate margin the highest averaged
current consumption value specified in the SARA-R4 series data sheet [1].
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 6.
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 Table 6:
• 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 current consumption occurring during transmissions at the
maximum power, as specified in the SARA-R4 series data sheet [1].
• 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 profile and therefore negatively impact modulation spectrum performance.
• PWM mode operation: it is preferable to select regulators with Pulse Width Modulation (PWM)
mode. While in connected mode, the Pulse Frequency Modulation (PFM) mode and PFM/PWM
modes transitions must be avoided to reduce noise on VCC voltage profile. Switching regulators
can be used that are able to switch between low ripple PWM mode and high ripple PFM mode,
provided that the mode transition occurs when the module changes status from the active mode
to connected mode. It is permissible to use a regulator that switches from the PWM mode to the
burst or PFM mode at an appropriate current threshold.
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