AN-1048: EnerChip™ CC as Backup Power for a DS1340 RTC

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Doc AN-72-1048 Rev A

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All of these features have been combined to create a backup power solution that eliminates the need for a

dedicated supply supervisory circuit, battery holder or socket, separate battery management circuitry, and a

conventional power source such as a coin cell or supercapacitor. The result is a compact, single-chip solution to

backup and bridging power. The power source - the EnerChip - is a component class, rechargeable solid state

battery that has no organic solvents or harmful chemicals that accompany coin cells and supercapacitors.

Given the low operating current of many RTCs, the EnerChip CC can provide several hours to several days

of backup time. Backup time for the DS1340 is calculated based on the typical operating current of 800nA

in RTC backup mode (25nA data retention current), plus the DS1340 battery leakage current of 100nA and

the EnerChip CC battery cutoff circuit current of 30nA (typical). The following table illustrates the amount of

backup time afforded by the EnerChip CC devices - CBC3105, CBC3112, and CBC3150 - when connected to the

DS1340 RTC.

EnerChip CC

DFN/QFN Package Size

(mm)

RTC Backup Time

(hours)

Data Retention Time

(hours)

CBC3105

5 x 5 x 0.9

6

32

CBC3112

7 x 7 x 0.9

14

76

CBC3150

9 x 9 x 0.9

60

320

Given that the vast majority power interruptions - whether due to grid power outages in wall-powered systems

or in mobile devices during main battery recharging and replacement - are less than one day, either EnerChip

CC device can be used in most applications. Once the minimum backup run time specification is defined,

device selection can be made accordingly and in conjunction with other considerations such as package

footprint. The EnerChip CC is also configured to allow additional surface mount EnerChips to be connected to

increase backup run time.

In cases where an MCU must also be maintained in a low power state (as opposed to being completely

disconnected from the circuit) during battery-backed operation, the MCU sleep current must be factored into

the backup run time equation. To maintain favorable backup run time, the designer can choose from a variety

of MCUs, some having sleep currents as low as 20nA. When using MCUs with such exceptionally low sleep

currents, the backup run time is not reduced significantly. During operation, the MCU can also be programmed

to drive the EnerChip CC ENABLE line low when EnerChip battery charging is not required. Doing so will reduce

the overhead current associated with operating the charge pump internal to the EnerChip CC.

Conclusion

Many systems utilizing RTCs require power supply backup in the event of main power interruption. Conventional

backup power sources are primary coin cells, rechargeable coin cells, and supercapacitors. Traditional backup

power sources have long charge times, high self-discharge, require relatively high steady state parasitic

charging current, and often require external components for charge control and discharge cutoff. The EnerChip

- with its low self-discharge, low parasitic current draw when charged, and fast recharge time - is a superior

solution to supercapacitors and coin cell batteries in backup power applications. When implemented in

systems using real-time clocks, the EnerChip CC delivers a robust backup power source with high cycle life and

integrated battery management in a low profile, compact surface mount package.