Integration Manual

SARA-R5 series - System integration manual
UBX-19041356 - R03 Design-in Page 38 of 123
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2.2 Supply interfaces
2.2.1 Module supply (VCC)
2.2.1.1 General guidelines for VCC supply circuit selection and design
All the available VCC pins have to be connected to the external supply minimizing the power loss due
to series resistance.
GND pins are internally connected. Application design shall connect all the available pads 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-R5 series modules must be sourced through the VCC pins with a suitable DC power supply that
should comply with the module VCC requirements summarized in Table 5.
The appropriate DC power supply can be selected according to the application requirements (see
Figure 19) 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 19: VCC supply concept selection
The switching step-down regulator is the typical choice when primary supply source has a nominal
voltage much higher (e.g. greater than 5 V) than the operating supply voltage of SARA-R5 series. The
use of switching step-down provides the best power efficiency for the overall application and
minimizes current drawn from the main supply source. See section 2.2.1.2 for design-in.
The use of an LDO linear regulator becomes convenient for a primary supply with a relatively low
voltage (e.g. less or equal 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. See section 2.2.1.3 for design-in.
If SARA-R5 series 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. See sections 2.2.1.4, 2.2.1.5, 2.2.1.6 and 2.2.1.7 for specific
design-in.