LISA-U series - System Integration Manual
UBX-13001118 - R17
Advance information
System description
Page 24 of 190
selected regulator or battery is not able to withstand the maximum
VCC
peak current, it must be able to
withstand at least the maximum average current consumption value specified in the module’ Data Sheet [1][2].
The additional energy required by the module during a GSM/GPRS Tx slot (when in the worst case the current
consumption can rise up to 2.5 A, as described in section 1.5.3.1) can be provided by an appropriate bypass tank
capacitor or supercapacitor with very large capacitance and very low ESR placed close to the module
VCC
pins.
Depending on the actual capability of the selected regulator or battery, the required capacitance can be
considerably larger than 1 mF and the required ESR can be in the range of few tens of m
. Carefully evaluate
the implementation of this solution since aging and temperature conditions significantly affect the actual
capacitor characteristics.
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
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
: select preferably 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 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 regulator must be
carefully evaluated, since the voltage at the
VCC
pins must ramp from 2.5 V to 3.2 V within 1 ms to allow a
proper switch-on of the module
Figure 7 and the components listed in Table 8 show an example of a high reliability power supply circuit, where
the module
VCC
is supplied by a step-down switching regulator capable of delivering 2.5 A current pulses 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.
