LISA-C2 series and FW75-C200 - System Integration Manual
UBX-13000620 - R21
Early Production Information
System description
Page 15 of 103
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-C200 and FW75-C200 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) batteries is uncommon, since most available batteries are seldom capable
of delivering the peak current 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-C200 and FW75-C200 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.
1.5.3.1
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 greater than
1.2 Amps for safe design margin.
•
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. An additional L-C low-pass filter
between the switching regulator output to
VCC
supply pins can mitigate the ripple on
VCC
, but adds extra
voltage drop due to resistive losses on series inductors.
•
PWM mode operation
: it is preferable to select regulators with Pulse Width Modulation (PWM) mode.
While in active mode, Pulse Frequency Modulation (PFM) mode, and PFM/PWM mode, transitions must be
avoided to reduce the noise on the
VCC
voltage profile. Switching regulators able to switch between low
ripple PWM mode and high efficiency burst or PFM mode can be used to provide the mode transition from
idle mode (current consumption approximately 2 mA) to active mode (current consumption approximately
100 mA). It is permissible to use a regulator that switches from the PWM mode to the burst or PFM mode at
an appropriate current threshold (e.g. 60 mA).
Figure 4 and the components listed in Table 6 show an example of a high reliability power supply circuit, where
the module
VCC
is supplied by a step-down switching regulator with low output ripple and with fixed switching
frequency in PWM mode operation greater than 1 MHz. The use of a switching regulator is suggested when the
difference from the available supply rail to the
VCC
value is high: switching regulators provide good efficiency
transforming a 12 V supply to the typical 3.8 V value of the
VCC
supply.
