NXP Semiconductors PCA8565, PCF85x3, PCF2123, PCA2125 Backup power supply, Lithium Primary cells

NXP Semiconductors	UM10301
	User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

13. Backup power supply

A real time clock is a clock that keeps track of the time as humans use it (hours, minutes, seconds, years etc.) and usually even when the rest of the system is turned off. Therefore in order to be able to always represent Real Time, real time clocks need a power supply even if the rest of the system is off. This backup power supply is often a dedicated battery or super capacitor. A super cap is a special low voltage capacitor that offers an unusual high capacitance of for example 0.47 F or 1 F in a relatively small package, especially developed for backing up volatile memory or RTCs. In case a battery is used, it may be a primary cell (non rechargeable) or a secondary cell (rechargeable) like NiCd or NiMH. All RTCs in this manual incorporate neither a dedicated switch-over circuit nor a charger and therefore this has to be realized with some external components. Only a few components are necessary to realize this as is illustrated in some example circuit diagrams.

If an RTC will be backed up by a battery or capacitor the current demands of the RTC, the required lifetime and the energy available in the backup source need to be matched. Backup source properties are dependent on the ambient conditions in which the application has to operate or will be stored and therefore it is important to consider these when making a choice of how to provide backup power. Criteria such as expected system life time, ambient temperature, manufacturing requirements, cost and legal regulations must be taken into account. The table below gives an indicative overview of possible backup power sources and key selection criteria.

Table 8. Overview of common backup supply components and key selection criteria

Technology	Operating	Self-	Charging circuit	Backup time	Cost	Restrictions on
	Temperature	discharge	and nr. of cycles			disposal and
	[° C]	rate				safety
Primary Lithium	-30 to +80	Low	n.a.	Long	Low	High
Rechargeable	0 to +40	Medium	Simple / ± 500	Short	Medium	Medium
(NiCd / NiMH)	(during charging)
Super Capacitor	-40 to +85	High	Simple / unlimited	Short	Medium / High	Low

13.1 Lithium Primary cells

Amongst the primary cells, the lithium battery has the highest energy density and a very low self discharge rate. This enables a long backup time without taking up too much space in the application. Lithium batteries, when not used properly, may constitute a risk of fire and therefore for the end product to get safety approval, certain guidelines must be taken into account. Refer to IEC/UL 60950.

Recognized lithium batteries are classified as either rechargeable or non-rechargeable. Non-rechargeable lithium batteries (primary cells) require two blocking components (diode) or a blocking component and a current limiting component (resistor) in its circuit. A rechargeable lithium battery (secondary cell) only requires a current limiting component. In order to charge such a battery properly, a relatively complicated circuit is necessary which controls both voltage and current and which will not be discussed here.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	30 of 52