Installation Instructions

LISA-U2 series - System Integration Manual
UBX-13001118 - R19 Early Production Information System description
Page 33 of 175
The internal regulator for V_BCKP is optimized for low leakage current and very light loads. It is not
recommended to use V_BCKP to supply external loads.
If V_BCKP is left unconnected and the module main voltage supply is removed from VCC, the RTC is supplied
from the bypass capacitor mounted inside the module. However, this capacitor is not able to provide a long
buffering time: within few milliseconds the voltage on V_BCKP will go below the valid range (1 V min). This has
no impact on cellular connectivity, as all the functionalities of the module do not rely on date and time setting.
Leave V_BCKP unconnected if the RTC is not required when the VCC supply is removed. The date and
time will not be 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.
If RTC 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]
= 2.50 x T [s] for LISA-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 will hold 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 will
allow the time reference to run during battery disconnection.
LISA-U2 series
C1
(a)
2
V_BCKP
R2
LISA-U2 series
C2
(superCap)
(b)
2
V_BCKP
D3
LISA-U2 series
B3
(c)
2
V_BCKP
Figure 16: 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
Reference
Description
Part Number - Manufacturer
C1
100 µF Tantalum Capacitor
GRM43SR60J107M - Murata
R2
4.7 kΩ Resistor 0402 5% 0.1 W
RC0402JR-074K7L - Yageo Phycomp
C2
70 mF Capacitor
XH414H-IV01E - Seiko Instruments
Table 14: Example of components for V_BCKP buffering
If longer buffering time is required to allow the 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 LISA-U2 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