User's Manual

LISA-U1/LISA-H1 series - System Integration Manual

3G.G2-HW-10002-2 Advance Information System description

Page 20 of 116

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

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, provided the mode transition occurs when the

GSM module changes status from idle mode (current consumption approximately 1 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)

 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 4 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.

LISA-U1/LISA-H1

series

12V

C3C2

VIN

RUN

SYNC

BOOST

GND

C8 C9

VCC

GND

Figure 7: Suggested schematic design for the VCC voltage supply application circuit using a step-down regulator