LISA-U2 series - System integration manual
UBX-13001118 - R27
System description
Page 34 of 183
C1-Public
for the detailed specification), whereas at +70 °C and an equal voltage, the power
consumption increases to 5-10 µA.
☞
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 [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.
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 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 k
Ω
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
