Ublox SARA-N2 Series, Руководство по системной интеграции

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SARA-N2 / N3 series - System integration manual
UBX-17005143 - R13 Design-in Page 40 of 95
C1-Public
2.2.2 RTC supply (V_BCKP)
2.2.2.1 Guidelines for V_BCKP circuit design
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The RTC supply ( V_BCKP pin) is not available on SARA-N2 series modules.
V_BCKP is the Real Time Clock (RTC) supply of SARA-N3 series modules. When VCC voltage is within
the valid operating range, the internal Power Management Unit (PMU) supplies the RTC and the same
supply voltage is available on the
V_BCKP pin.
If the VCC voltage is under the minimum operating limit (e.g. during non powered mode), the RTC can
be externally supplied through the V_BCKP pin, as for example using a suitable external capacitor, a
suitable supercapacitor, or a suitable non-rechargeable battery as illustrated in Figure 19 .
Resistor
SARA-N3 series
SuperCap
(b)
2
V_BCKP
Diode
SARA-N3 series
Battery
(c)
2
V_BCKP
SARA-N3 series
Cap
(a)
2
V_BCKP
Figure 19: V_BCKP application circuits using: (a) a capacitor, (b) a supercapacitor, (c) a non-rechargeable battery
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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 .
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The internal regulator for V_BCKP is optimized for low leakage current and very light loads. Do not
apply loads that might exceed the limit for maximum available current from V_BCKP supply, as
this can cause malfunction in the module.
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The V_BCKP pin provides internal short circuit protection to limit start-up current and protect the
device in short circuit situations. No additional external short circuit protection is required.
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The ESD sensitivity rating of the V_BCKP pin is 1 kV (Human Body Model according to JESD22-
A114). Higher protection level could be required if the line is externally accessible on the application
board. Higher protection level can be achieved by mounting an external ESD protection (e.g.
EPCOS CA05P4S14THSG varistor array) close to accessible point.
2.2.2.2 Guidelines for V_BCKP layout design
The RTC supply ( V_BCKP ) requires careful layout: avoid injecting noise on this voltage domain as it
may affect the stability of the 32 kHz oscillator