Integration Manual

LISA-U series - System Integration Manual
3G.G2-HW-10002-A3 Preliminary System description
Page 22 of 160
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 RESET_N 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-U series modules must be supplied through the VCC pins by one (and only one) proper DC power supply
that must be one of 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
>5 V?
Switching
Step-Down
Regulator
No, portable device
No, less than 5 V
Yes, greater than 5 V
Yes, always available
Figure 6: 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-U series modules 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.
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-U 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 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.
Keep in mind that the use of batteries requires the implementation of a suitable charger circuit (not included in
LISA-U 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.