TOBY-L4 series - System Integration Manual
UBX-16024839 - R04
Design-in
Page 65 of 143
2.2.1.10
Guidelines for VCC supply layout design
A clean 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 RF lines / parts, sensitive analog signals
and sensitive functional units: it is good practice to interpose at least one layer of PCB ground between the
VCC
track and other signal routing.
Coupling between
VCC
and digital lines, especially USB, must be avoided.
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 module tank capacitor.
The bypass capacitors in the pF range illustrated in Figure 24 and Table 17 should be placed as close as
possible to the
VCC
pins, where the
VCC
line narrows close to the module input pins, improving the RF
noise rejection in the band centered on the Self-Resonant Frequency of the pF capacitors. This is highly
recommended if the application device integrates an internal antenna.
Since
VCC
input provides the supply to the RF Power Amplifiers, any 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 TOBY-L4 series modules in the worst case.
Shielding of 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 the
VCC
is protected by a 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 module, preferably
closer to the DC source (otherwise protection functionality may be compromised).
2.2.1.11
Guidelines for grounding layout design
A clean 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
pad surrounding the
VCC
pins has 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 the 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.
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 the
GND
pads also ensures the thermal heat sink. This is critical during connection,
when the real network commands the module to transmit at maximum power: correct grounding helps
prevent module overheating.
