SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R08
Objective Specification
Design-in
Page 93 of 188
2.2.2
RTC supply (V_BCKP)
2.2.2.1
Guidelines for V_BCKP circuit design
If RTC timing is required to run for a time interval of T [s] at 25 °C when
VCC
supply is removed, place a
capacitor with a nominal capacitance of C [µF] at the
V_BCKP
pin. Choose the capacitor using the following
formula:
C [µF] = (Current_Consumption [µA] x T [s]) / Voltage_Drop [V]
= 1.5 x T [s] for SARA-G3 series
= 2.5 x T [s] for SARA-U2 series
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 70 s (SARA-G3 series)
or for around 40 s (SARA-U2 series) 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 ~13 hours
(SARA-G3 series) or for ~8 hours (SARA-U2 series) 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
SARA-U2 series
C1
(a)
2
V_BCKP
R2
SARA-G3 series
SARA-U2 series
C2
(superCap)
(b)
2
V_BCKP
D3
SARA-G3 series
SARA-U2 series
B3
(c)
2
V_BCKP
Figure 43: Real time clock supply (V_BCKP) application circuits: (a) using a 100 µF capacitor to let the RTC run for ~1 minute 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
Reference
Description
Part Number - Manufacturer
C1
100 µF Tantalum Capacitor
GRM43SR60J107M - Murata
R2
4.7 k
Resistor 0402 5% 0.1 W
RC0402JR-074K7L - Yageo Phycomp
C2
70 mF Capacitor
XH414H-IV01E - Seiko Instruments
Table 22: 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 the
SARA-G3
[1] or in the
SARA-U2 series
Data Sheet
[2]). 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
