TOBY-L4 series - System Integration Manual
UBX-16024839 - R04
Design-in
Page 54 of 143
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 2.2.1.7, 2.2.1.8, and 2.2.1.4, 2.2.1.6, 2.2.1.10, 2.2.1.11 for specific design-in.
An appropriate primary (not rechargeable) battery can be selected taking into account the maximum current
specified in the
TOBY-L4 series Data Sheet
[1] during connected mode, considering that primary cells might have
weak power capability. See sections 2.2.1.5, and 2.2.1.6, 2.2.1.10, 2.2.1.11 for specific design-in.
The usage of more than one DC supply at the same time should be evaluated carefully: 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-L4 series Data Sheet
[1] 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-L4 series Data Sheet
[1]. 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 Table 7.
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 7:
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-L4 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 evaluated carefully since this can produce noise in the
VCC
voltage profile and therefore
negatively impact LTE/3G/2G modulation spectrum performance. An additional L-C low-pass filter between
the switching regulator output to
VCC
supply pins can mitigate the ripple at the input of the module, but
adds extra voltage drop due to resistive losses on series inductors.
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 idle/active modes 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.
