User's Guide

LE920A4 HW User Guide Power Supply
Doc#: 1VV0301261 General Design Rules
Rev. 4.7.1 Page 57 of 124 2019-11-21
Figure 11 shows an example of a linear regulator with 5V input.
Figure 11: Linear Regulator with 5V Input
6.2.1.2. + 12V Input Source Power Supply Design Guidelines
 The desired output for the power supply is 3.8V. Due to the big difference between
the input source and the desired output, a linear regulator is unsuitable and must
not be used. A switching power supply is preferable because of its better
efficiency, especially with the 2A peak current load which is expected during GSM
Tx.
 When using a switching regulator, a 500-kHz or higher switching frequency
regulator is preferable because of its smaller inductor size and its faster transient
response. This allows the regulator to respond quickly to the current peaks
absorption.
 In any case, the selection of the frequency and switching design is related to the
application to be developed due to the fact that the switching frequency can also
generate EMC interference.
 For car batteries (lead-acid accumulators) the input voltage can rise up to 15.8V.
This must be kept in mind when choosing components: all components in the
power supply must withstand this voltage.
 A bypass low ESR capacitor of adequate capacity must be provided to cut the
current absorption peaks. A 100μF tantalum capacitor is usually suitable (on both
VBATT and VBATT_PA together).
 Make sure that the low ESR capacitor on the power supply output (usually a
tantalum one) is rated at least 10V.
 For automotive applications, a spike protection diode must be inserted close to the
power input to clean the supply of spikes.
 A protection diode must be inserted close to the power input to protect the module
from power polarity inversion. This can be the same diode as for spike protection.