SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 113 of 217
Combining a SARA-G3 or a SARA-U2 cellular module with a u-blox GNSS positioning receiver, the positioning
receiver
VCC
supply is controlled by the cellular module by means of the “GNSS supply enable” function
provided by the
GPIO2
of the cellular module. In this case the
V_BCKP
supply output of the cellular module can
be connected to the
V_BCKP
backup supply input pin of the GNSS receiver to provide the supply for the
positioning real time clock and backup RAM when the
VCC
supply of the cellular module is within its operating
range and the
VCC
supply of the GNSS receiver is disabled. This enables the u-blox GNSS receiver to recover
from a power breakdown with either a hot start or a warm start (depending on the duration of the positioning
VCC
outage) and to maintain the configuration settings saved in the backup RAM. Refer to section 2.6.4 for
more details regarding the application circuit with a u-blox GNSS receiver.
On SARA-G300 and SARA-G310 modules, the
V_BCKP
supply output can be used to supply an external 32 kHz
oscillator which provides a 32 kHz signal to the
EXT32K
input pin as reference clock for the RTC timing, so that
the modules can enter the low power idle mode and can make available the RTC functions.
The internal regulator for
V_BCKP
is optimized for low leakage current and very light loads. Do not apply
loads which might exceed the limit for maximum available current from
V_BCKP
supply, as this can cause
malfunctions in the module. The
SARA-G3
[1] and the
SARA-U2 series Data Sheet
describe the detailed electrical characteristics.
V_BCKP
supply output pin provides internal short circuit protection to limit start-up current and protect the
device in short circuit situations. No additional external short circuit protection is required.
The ESD sensitivity rating of the
V_BCKP
supply pin is 1 kV (Human Body Model according to JESD22-
A114). A higher protection level can be required if the line is externally accessible on the application
board, e.g. if an accessible back-up battery connector is directly connected to
V_BCKP
pin, and it can be
achieved by mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) close to accessible
points.
2.2.2.2
Guidelines for V_BCKP layout design
RTC supply (
V_BCKP
) requires careful layout: avoid injecting noise on this voltage domain as it may affect the
stability of the 32 kHz oscillator.
