LISA-U2 series - System integration manual
UBX-13001118 - R27
System description
Page 20 of 183
C1-Public
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The voltage at the
VCC
pins must ramp from 2.5 V to 3.2 V within 1 ms. This
VCC
slope allows a
proper switch-on of the module when the voltage rises to the
VCC
normal operating range from a
voltage of less than 2.25 V. If the external supply circuit cannot raise the
VCC
voltage from 2.5 V
to 3.2 V within 1 ms, the
RESET_N
pin should be kept low during
VCC
rising edge, so that the
module will switch on releasing the
RESET_N
pin when the
VCC
voltage stabilizes at its nominal
value within the normal operating range.
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
) 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.
