LENA-R8 series - System integration manual
UBX-22015376 - R02
Design-in
Page 47 of 116
C1-Public
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 improves RF and thermal heat sink figures 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 PCB 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 PCB. Use as many vias as possible to connect ground planes.
•
Provide a dense line of vias at the edges of each GND 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.
2.2.2
Cellular RTC supply (V_BCKP)
2.2.2.1
Guidelines for V_BCKP circuit design
LENA-R8 series cellular system provides the
V_BCKP
RTC supply input/output for the cellular system,
which can be mainly used to:
•
Provide cellular system RTC back-up when
VCC
supply is removed
External power supply at
V_BCKP
is optional. Keeping the RTC of the cellular system running when
VCC
supply is removed is not required for most of the applications, so that the
V_BCKP
pin can be left
unconnected in most of the applications.
In case the end-device application requires to keep the RTC of the cellular system running when the
main
VCC
supply is removed, one of the following possible alternative options may be considered:
(a)
70 mF super-capacitor placed at
V_BCKP
, with a 4.7 k
series resistor, to let the cellular RTC run
after
VCC
removal
(b)
Back-up battery placed at
V_BCKP
, with a series diode, to let the cellular RTC run for much longer
time after
VCC
removal
R1
LENA-R8 series
C1
(superCap)
(a)
2
V_BCKP
D1
LENA-R8 series
B1
(b)
2
V_BCKP
Figure 29: LENA-R8 series cellular system Real Time Clock supply (V_BCKP) application circuits
Reference
Description
Part number - manufacturer
R1
4.7 k
Resistor 0402 5% 0.1 W
RC0402JR-074K7L - Yageo Phycomp
C1
70 mF Capacitor
XH414H-IV01E - Seiko Instruments
Table 17: Example of components for V_BCKP buffering
