User's Manual

LISA-U1/LISA-H1 series - System Integration Manual
3G.G2-HW-10002-2 Advance Information System description
Page 19 of 116
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-U1/LISA-H1 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-U1/LISA-H1 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 Lithium-Ion 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-U1/LISA-H1 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 following sections highlight some design aspects for each of the supplies listed above.
Switching regulator
The characteristics of the switching regulator connected to VCC pins should meet the following requirements:
 Power capability: the switching regulator with its output circuit must be capable of providing a voltage
value to the VCC pins within the specified operating range and must be capable of delivering 2.5 A current
pulses with 1/8 duty cycle to the VCC pins
 Low output ripple: the switching regulator together with its output circuit must be capable of providing a
clean (low noise) VCC voltage profile
 High switching frequency: for best performance and for smaller applications select a switching frequency
≥ 600 kHz (since L-C output filter is typically smaller for high switching frequency). The use of a switching
regulator with a variable switching frequency or with a switching frequency lower than 600 kHz must be
carefully evaluated since this can produce noise in the VCC voltage profile and therefore negatively impact
GSM modulation spectrum performance. An additional L-C low-pass filter between the switching regulator