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2.2.1.11 Guidelines for VCC supply layout design
Good connection of the module VCC pins with a DC supply source is required for correct RF performance.
Guidelines are summarized in the following list:
All the available VCC pins must be connected to the DC source.
VCC connection must be as wide as possible and as short as possible.
Any series component with Equivalent Series Resistance (ESR) greater than few milliohms must be avoided.
VCC connection must be routed through a PCB area separated from sensitive analog signals and sensitive
functional units: it is good practice to interpose at least one layer of PCB ground between VCC track and other
signal routing.
Coupling between VCC and audio lines (especially microphone inputs) must be avoided, because the typical
GSM burst has a periodic nature of approximaely 217 Hz, which lies in the audible audio range.
The tank bypass capacitor with low ESR for current spikes smoothing described in section 2.2.1.6 should be
placed close to the VCC pins. If the main DC source is a switching DC-DC converter, place the large capacitor
close to the DC-DC output and minimize the VCC track length. Otherwise, consider using separate capacitors
for the DC-DC converter and the cellular module tank capacitor.
The bypass capacitors in the pF range described in section 2.2.1.6 should be placed as close as possible to the
VCC pins. This is highly recommended if the application device integrates an internal antenna.
Since VCC is directly connected to RF Power Amplifiers, voltage ripple at high frequency may result in
unwanted spurious modulation of the transmitter RF signal. This is more likely to happen with switching
DC-DC converters, in which case it is better to select the highest operating frequency for the switcher and
add a large L-C filter before connecting to the LARA-R2 series modules in the worst case.
Shielding of the switching DC-DC converter circuit, or at least the use of shielded inductors for the switching
DC-DC converter, may be considered since all switching power supplies may potentially generate interfering
signals as a result of high-frequency, high-power switching.
If VCC is protected by transient voltage suppressor to ensure that the voltage maximum ratings are not
exceeded, place the protecting device along the path from the DC source toward the cellular module,
preferably closer to the DC source (otherwise protection functionality may be compromised).
2.2.1.12 Guidelines for grounding layout design
Good connection of the module GND pins with the application board solid ground layer is required for correct RF
performance. It significantly reduces EMC issues and provides a thermal heat sink for the module.
Connect each GND pin with the application board solid GND layer. It is strongly recommended that each GND
pin surrounding VCC pins have one or more dedicated via down to the application board solid ground layer.
The VCC supply current flows back to the main DC source through GND as ground current: provide an
adequate return path with a suitable uninterrupted ground plane to the main DC source.
It is recommended to implement one layer of the application board as a ground plane as wide as possible.
If the application board is a multilayer PCB, then all the board layers should be filled with GND plane as much
as possible and each GND area should be connected together with a complete via stack down to the main
ground layer of the board. Use as many vias as possible to connect the ground planes
Provide a dense line of vias at the edges of each ground area, in particular along RF and high speed lines
If the whole application device is composed of more than one PCB, then it is required to provide a good and
solid ground connection between the GND areas of all the multiple PCBs.
Good grounding of GND pins also ensures thermal heat sink. This is critical during call connection, when the
real network commands the module to transmit at maximum power: proper grounding helps prevent module
overheating.
UBX-16010573 - R12
LARA-R2 series - System Integration Manual
Design-in
Page 81 of 157
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