User's Manual
Note d‘étude / Technical document :
URD1– OTL 5696.1– 003 / 72361 Edition 02
HILO 3G Application Note
17 January 2011 - Page 23 / 38
3.8 Backup Battery
3.8.1 Backup Battery Function Feature
3.8.1.1 With Backup Battery
A backup battery can be connected to the module in order to supply internal RTC (Real Time Clock) when the main power
supply is removed. Thus, when the main power supply is removed, the RTC is still supplied with power and the module
keeps the time register running.
• If VBAT < 3V, internal RTC is supplied by VBACKUP.
• If VBAT ≧3V, internal RTC is supplied by VBAT.
3.8.1.2 Without Backup Battery
Without backup battery
If VBAT ≥ 1.5V, internal RTC is supplied by VBAT.
If VBAT < 1.5V, internal RTC is not supplied.
To calculate the backup battery capacity; consider that current consumption for RTC on the backup battery is up to 2µA
depending on the temperature.
Pin Name
Min
Max
VBACKUP
2uA
3.8.2 Charge by Internal HiLo3G Charging Function
The charging function is available on HiLo3G without any additional external power supply (the charging power supply is
provided by the HiLo3G).
Charge of the back-up battery occurs only when main power supply VBAT is provided.
Figure 23: Backup battery or 10μF capacitor internally charged
The resistor R depends on the charging current value provided by the battery manufacturer.
3.8.3 Backup Battery Technology
3.8.3.1 Capacitor Battery
These kinds of backup battery do not have the drawbacks of the Lithium Ion rechargeable battery.
As there are only capacitors:
• The maximum discharge current is generally bigger,
• There is no need to regulate the charging current.
Moreover, this kind of battery is available in the same kind of package than the Lithium Ion cell and fully compatible on a
mechanical perspective. The only disadvantage is that the capacity of this kind of battery is significantly smaller than
Manganese Silicon Lithium Ion battery. But for this kind of use (supply internal RTC when the main battery is removed),
the capacity is generally enough.