Integration Manual

TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R04 Advance Information Design-in
Page 72 of 141
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 [µF] at the V_BCKP pin. Choose the capacitor using the following formula:
C [µF] = (Current_Consumption [µA] x T [s]) / Voltage_Drop [V]
= 1.25 x T [s]
For example, a 100 µF 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 35: Real time clock supply (V_BCKP) application circuits: (a) using a 100 µF 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 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 20: 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 [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 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.