Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 System description Page 20 of 182
1.5.2.1 VCC application circuits
LISA-U2 series modules must be supplied through the VCC pins by a clean DC power supply, which
can be selected according to the application requirements (see Figure 5) between the different
possible supply sources types, the most common ones of which 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 5: VCC supply concept selection
The 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 LISA-U2 series module’s operating
supply voltage. The use of switching step-down provides the best power efficiency for the overall
application and minimizes the current drawn from the main supply source.
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 will
diminish the benefit of voltage step-down and no true advantage will be gained in input current
savings. On the opposite side, linear regulators are not recommended for high voltage step-down as
they will dissipate a considerable amount of energy in thermal power.
If LISA-U2 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.
The use of a primary (not rechargeable) battery is uncommon, since most of the cells available are
seldom capable of delivering the burst peak current for a GSM call due to high internal resistance.
Keep in mind that the use of batteries requires the implementation of a suitable charger circuit (not
included in LISA-U2 series modules). The charger circuit should be designed in order to prevent over-
voltage on VCC beyond the upper limit of the absolute maximum rating.
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 being mutually exclusive.
The usage of a regulator or a battery not able to withstand the maximum VCC peak current
consumption stated in the LISA-U2 series Data Sheet [1] is generally not recommended. However, if
the selected regulator or battery is not able to withstand the maximum VCC peak current, it must be