Integration Manual

LISA-U2 series - System Integration Manual
UBX-13001118 - R25 System description Page 34 of 182
The RTC has very low power consumption, but is highly temperature dependent. For example at
+25 °C, with the V_BCKP voltage equal to the typical output value, the power consumption is
approximately 2 µ A (see the input characteristics of supply/power pins table in the LISA-U2 series
Data Sheet [1] for the detailed specification), whereas at +70 °C and an equal voltage, the power
consumption increases to 5-10 µ A.
☞ 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 a few milliseconds, the voltage on V_BCKP will drop below the
valid range (1 V minimum). This has no impact on cellular connectivity, as none of the functionalities
of the module rely on the date and time settings.
☞ 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 [µ F] at the V_BCKP pin. Choose the capacitor using the
following formula:
C [µ F] = (Current_Consumption [µ A] x T [s]) / Voltage_Drop [V]
= 2.50 x T [s] for LISA-U2 series
For example, a 100 µ F 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.
1:1 scaling
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 µ F 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 µ F 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