SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 100 of 217
2.2.1.2
Guidelines for VCC supply circuit design using a switching regulator
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 or greater voltage supply to the typical
3.8 V value of the
VCC
supply.
The characteristics of the switching regulator connected to
VCC
pins should meet the following prerequisites to
comply with the module
VCC
requirements summarized in Table 6:
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 to
VCC
pins
the specified maximum peak / pulse current with 1/8 duty cycle (see the
SARA-G3
[1] or
the
SARA-U2 series Data Sheet
[2]).
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
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 connected 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 that are able to switch
between low ripple PWM mode and high efficiency burst or PFM mode can be used, provided the mode
transition occurs when the module changes status from idle/active mode to connected mode (where current
consumption increases to a value greater than 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).
Output voltage slope
: the use of the soft start function provided by some voltage regulators should be
carefully evaluated, since the
VCC
pins voltage must ramp from 2.5 V to 3.2 V in less than 1 ms to switch on
the SARA-U2 modules or in less than 4 ms to switch on the SARA-G3 modules by applying
VCC
supply, that
otherwise can be switched on by forcing a low level on the
RESET_N
pin during the
VCC
rising edge and
then releasing the
RESET_N
pin when the
VCC
supply voltage stabilizes at its proper nominal value.
