Integration Manual

LISA-U series - System Integration Manual
3G.G2-HW-10002-A3 Preliminary System description
Page 34 of 160
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-U series
C1
(a)
2
V_BCKP
R2
LISA-U series
C2
(superCap)
(b)
2
V_BCKP
D3
LISA-U 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 13: 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-U1 series Data Sheet [1] and in LISA-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.
Combining a LISA-U series wireless module with a u-blox GPS receiver, the VCC supply of the GPS receiver is
controlled by the wireless module by means of the GPS supply enable function provided by the GPIO2 of the
wireless module. In this case the V_BCKP supply output of the LISA-U series wireless module can be connected
to the V_BCKP backup supply input pin of the GPS receiver to provide the supply for the GPS real time clock and
backup RAM when the VCC supply of the wireless module is within its operating range and the VCC supply of
the GPS receiver is disabled. This enables the u-blox GPS receiver to recover from a power breakdown with either
a Hotstart or a Warmstart (depending on the duration of the GPS VCC outage) and to maintain the
configuration settings saved in the backup RAM. Refer to section 1.10 for more details regarding the application
circuit with a u-blox GPS receiver.
1.5.5 Interface supply (V_INT)
The same voltage domain used internally to supply the digital interfaces is also available on the V_INT pin. The
internal regulator that generates the V_INT supply is a switching step down converter that is directly supplied
from VCC. The voltage regulator output is set to 1.8 V (typical) when the module is switched on and is disabled
when the module is switched off or when the RESET_N pin is forced the low level. The switching regulator
operates in Pulse Width Modulation (PWM) for high output current mode but automatically switches to Pulse