SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26
Design-in
Page 112 of 217
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 seconds (SARA-G3
series) or for around 40 seconds (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 51: 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 32: 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
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
.
