Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 System description Page 21 of 182
able to withstand at least the maximum average current consumption value specified in the LISA-U2
series Data Sheet [1].
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 a 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.