TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R28
Design-in
Page 70 of 164
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 in the
application 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
The use of a primary (not rechargeable) battery is in general uncommon, but appropriate parts can be
selected given that the most cells available are seldom capable of delivering the maximum current
specified in TOBY-L2 series Data Sheet
during connected-mode. Carefully evaluate the usage of
super-capacitors as supply source since aging and temperature conditions significantly affect the
actual capacitor characteristics. See
for specific design-in.
Rechargeable 3-cell Li-Ion or Li-Pol and Ni-MH chemistry batteries reach a maximum voltage that is
above the maximum rating for the 3.3Vaux supply of MPCI-L2 modules, and should be avoided. The
use of rechargeable, not-rechargeable battery or super-capacitors is very uncommon for Mini PCI
Express applications, so that these supply sources types are not considered for MPCI-L2 modules.
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 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 supply circuit design using a switching regulator
The use of a switching regulator is suggested when the difference from the available supply rail to the
VCC or the 3.3Vaux 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 or the typical 3.3 V value of
the 3.3Vaux supply.
The characteristics of the switching regulator connected to VCC or 3.3Vaux pins should meet the
following prerequisites to comply with module VCC or 3.3Vaux requirements summarized in
Power capability: the switching regulator with its output circuit must be capable of providing a
voltage value to the VCC or 3.3Vaux pins within the specified operating range and must be capable
of delivering to VCC or 3.3Vaux pins the maximum peak / pulse current consumption during Tx
burst at maximum Tx power specified in the TOBY-L2 or MPCI-L2 Data Sheet
Low output ripple: the switching regulator together with its output circuit must be capable of
providing a clean (low noise) VCC or 3.3Vaux 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 or 3.3Vaux 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 or 3.3Vaux 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 the noise on the VCC or 3.3Vaux 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
