User Manual

LARA-R2 series - System Integration Manual
UBX-16010573 - R02 Objective Specification Design-in
Page 79 of 148
2.2.2 RTC supply (V_BCKP)
2.2.2.1 Guidelines for V_BCKP circuit design
LARA-R2 series modules provide the V_BCKP RTC supply input/output, which can be mainly used to:
 Provide RTC back-up when VCC supply is removed
If RTC timing is required to run for a time interval of T [s] 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]
= 2.5 x T [s]
For example, a 100 µF capacitor can be placed at V_BCKP to provide RTC backup holding the V_BCKP voltage
within its valid range for around 40 s at 25 °C, after the VCC supply is removed. If a longer buffering time is
required, a 70 mF super-capacitor 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 8 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.
LARA-R2 series
C1
(a)
2
V_BCKP
R2
LARA-R2 series
C2
(superCap)
(b)
2
V_BCKP
D3
LARA-R2 series
B3
(c)
2
V_BCKP
Figure 36: 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 28: 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 LARA-R2 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.