User's Manual

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08 Objective Specification Design-in
Page 82 of 188
2.2 Supply interfaces
2.2.1 Module supply (VCC)
2.2.1.1 General guidelines for VCC supply circuit selection and design
VCC pins are internally connected, but connect all the available pins to the external supply to minimize the
power loss due to series resistance.
GND pins are internally connected but connect all the available pins to solid ground on the application board,
since a good (low impedance) connection to external ground can minimize power loss and improve RF and
thermal performance.
SARA-G3 and SARA-U2 series modules must be supplied through the VCC pins by a proper DC power supply
that should comply with the module VCC requirements summarized in Table 7.
The proper DC power supply can be selected according to the application requirements (see Figure 35) between
the different possible supply sources types, which most common ones are the following:
 Switching regulator
 Low Drop-Out (LDO) linear regulator
 Rechargeable Lithium-ion (Li-Ion) or Lithium-ion polymer (Li-Pol) battery
 Primary (disposable) battery
Main Supply
Available?
Battery
Li-Ion 3.7 V
Linear LDO
Regulator
Main Supply
Voltage > 5V?
Switching Step-Down
Regulator
No, portable device
No, less than 5 V
Yes, greater than 5 V
Yes, always available
Figure 35: VCC supply concept selection
The DC/DC switching step-down regulator is the typical choice when the available primary supply source has a
nominal voltage much higher (e.g. greater than 5 V) than the modules VCC operating supply voltage. The use of
switching step-down provides the best power efficiency for the overall application and minimizes current drawn
from the main supply source. Refer to sections 2.2.1.2 and 2.2.1.6, 2.2.1.9, 2.2.1.10 for specific design-in.
The use of an LDO linear regulator becomes convenient for a primary supply with a relatively low voltage (e.g.
less than 5 V). In this case the typical 90% efficiency of the switching regulator diminishes the benefit of voltage
step-down and no true advantage is gained in input current savings. On the opposite side, linear regulators are
not recommended for high voltage step-down as they dissipate a considerable amount of energy in thermal
power. Refer to sections 2.2.1.3 and 2.2.1.6, 2.2.1.9, 2.2.1.10 for specific design-in.
If the modules are deployed in a mobile unit where no permanent primary supply source is available, then a
battery will be required to provide VCC. A standard 3-cell Li-Ion or Li-Pol battery pack directly connected to VCC
is the usual choice for battery-powered devices. During charging, batteries with Ni-MH chemistry typically reach
a maximum voltage that is above the maximum rating for VCC, and should therefore be avoided. Refer to
sections 2.2.1.4 and 2.2.1.6, 2.2.1.9, 2.2.1.10 for specific design-in.
The use of a primary (not rechargeable) battery is uncommon, since the most cells available are seldom capable
of delivering the burst peak current for a GSM call due to high internal resistance. Refer to sections 2.2.1.5 and
2.2.1.6, 2.2.1.9, 2.2.1.10 for specific design-in.