SARA-G3 series - System Integration Manual
UBX-13000995 - R06 Objective Specification Design-in
Page 102 of 218
For example, a 100 µF capacitor (such as the Murata GRM43SR60J107M) can be placed at V_BCKP
to provide a long buffering time. This capacitor holds V_BCKP voltage within its valid range for around 50
s at 25 °C, after the VCC supply is removed. If a very long buffering time is required, a 70 mF super-
capacitor (e.g. Seiko Instruments XH414H-IV01E) can be placed at V_BCKP, with a 4.7 k series
resistor to hold the V_BCKP voltage within its valid range for approximately 10 hours at 25 °C, after the
VCC supply is removed. The purpose of the series resistor is to limit the capacitor charging current due
to the large capacitor specifications, and also to let a fast rise time of the voltage value at the V_BCKP
pin after VCC supply has been provided. These capacitors allow the time reference to run during battery
disconnection.
SARA-G3 series
C1
(a)
2
V_BCKP
R2
SARA-G3 series
C2
(superCap)
(b)
2
V_BCKP
D3
SARA-G3 series
B3
(c)
2
V_BCKP
Figure 32: Real time clock supply (V_BCKP) application circuits: (a) using a 100 µF capacitor to let the RTC run for ~50 s after VCC removal;
(b) using a 70 mF capacitor to let RTC run for ~10 hours after VCC removal; (c) using a non-rechargeable battery
Part Number - Manufacturer
100 µF Tantalum Capacitor
4.7 kΩ Resistor 0402 5% 0.1 W
RC0402JR-074K7L - Yageo Phycomp
XH414H-IV01E - Seiko Instruments
Table 18: Example of components for V_BCKP buffering
If longer buffering time is required to allow the RTC time reference to run during a disconnection of the
VCC supply, then an external battery can be connected to V_BCKP pin. The battery should be able to
provide a proper nominal voltage and must never exceed the maximum operating voltage for V_BCKP
(specified in the Input characteristics of Supply/Power pins table in
SARA-G3 series Data Sheet
[1]).
The connection of the battery to V_BCKP should be done with a suitable series resistor for a
rechargeable battery, or with an appropriate series diode for a non-rechargeable battery. The purpose of
the series resistor is to limit the battery charging current due to the battery specifications, and also to
allow a fast rise time of the voltage value at the V_BCKP pin after the VCC supply has been provided.
The purpose of the series diode is to avoid a current flow from the module V_BCKP pin to the non-
rechargeable battery.
If the RTC timing is not required when the VCC supply is removed, it is not needed to connect
the V_BCKP pin to an external capacitor or battery. In this case the date and time are not
updated when VCC is disconnected. If VCC is always supplied, then the internal regulator is
supplied from the main supply and there is no need for an external component on V_BCKP.
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