TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R28
Design-in
Page 81 of 164
2.2.2
RTC supply output (V_BCKP)
☞
The RTC supply V_BCKP pin is not available on MPCI-L2 series modules.
2.2.2.1
Guidelines for V_BCKP circuit design
TOBY-L2 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 [
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]
= 1.25 x T [s]
For example, a 100
µ
F capacitor can be placed at V_BCKP to provide RTC backup holding the V_BCKP
voltage within its valid range for around 80 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 15 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.
TOBY-L2 series
C1
(a)
3
V_BCKP
R2
TOBY-L2 series
C2
(superCap)
(b)
3
V_BCKP
D3
TOBY-L2 series
B3
(c)
3
V_BCKP
Figure 41: Real time clock supply (V_BCKP) application circuits: (a) using a 100
µ
F capacitor to let the RTC run for ~80 s after
VCC removal; (b) using a 70 mF capacitor to let RTC run for ~15 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 k
Resistor 0402 5% 0.1 W
RC0402JR-074K7L – Yageo Phycomp
C2
70 mF Capacitor
XH414H-IV01E – Seiko Instruments
Table 22: Example of components for V_BCKP buffering
If very long 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 TOBY-L2 series Data
Sheet
). 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.
