TOBY-L4 series - System Integration Manual
UBX-16024839 - R04
Design-in
Page 66 of 143
2.2.2
RTC back-up supply (V_BCKP)
2.2.2.1
Guidelines for V_BCKP circuit design
TOBY-L4 series modules provide the
V_BCKP
pin, which can be used to:
power the Real Time Clock (RTC) only when the voltage value at
VCC
main module supply input is too low
The RTC can be supplied from an external back-up battery or capacitor through the
V_BCKP
pin, when the main
module voltage supply is not applied to the
VCC
pins. This lets the time reference (date and time) run as long as
the
V_BCKP
voltage is within its valid range, even when the main supply is not provided to the module.
Figure 29 and Table 21 describe possible application circuits for
V_BCKP
Real Time Clock (RTC) back-up supply:
a.
100 µF capacitor, to let the RTC run for ~1 minute after the
VCC
removal
b.
70 mF super-capacitor with a 4.7 k
series resistor, to let the RTC run for ~1 hour after the
VCC
removal
c.
Coin non-rechargeable battery with series diode, to let the RTC run for days after the
VCC
removal
TOBY-L4 series
C1
a.
3
V_BCKP
R2
TOBY-L4 series
C2
(SuperCap)
b.
3
V_BCKP
D3
TOBY-L4 series
B3
c.
3
V_BCKP
Figure 29: RTC back-up supply (V_BCKP) application circuits using a capacitor, a super-capacitor, or a non-rechargeable battery
Reference
Description
Part Number - Manufacturer
C1
100 µF 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 21: Example of components for RTC back-up supply (V_BCKP)
The
V_BCKP
supply output pin provides internal short circuit protection to limit the start-up current and protect
the device in short circuit situations. No additional external short circuit protection is required.
The
V_BCKP
pin can be left unconnected if the RTC timing is not required when the
VCC
supply is
removed.
The internal regulator for
V_BCKP
is optimized for low leakage current and very light loads. Do not apply
loads which might exceed the limit for the maximum available current from
V_BCKP
supply, as this can
cause malfunctions in the module. The
TOBY-L4 series
[1] describes the detailed electrical
characteristics.
The ESD sensitivity rating of the
V_BCKP
supply pin is 1 kV (HBM according to JESD22-A114). A higher
protection level can be required if the line is externally accessible on the application board, e.g. if an
accessible back-up battery connector is directly connected to the
V_BCKP
pin, and it can be achieved by
mounting an ESD protection (e.g. EPCOS CA05P4S14THSG varistor array) close to the accessible point.
2.2.2.2
Guidelines for V_BCKP layout design
The
V_BCKP
supply requires careful layout: avoid injecting noise on this voltage domain, as it may affect the
stability of the internal circuitry.