LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
System description
Page 20 of 125
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 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 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 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
