NXP Semiconductors UM10301 PCF2123 - Manual (Page 30)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 30 of 52
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
Temperature
[° C] Self-
discharge
rate Charging circuit
and nr. of cycles Backup time Cost Restrictions on
disposal and
safety
Primary Lithium -30 to +80 Low n.a. Long Low High
Rechargeable
(NiCd / NiMH) 0 to +40
(during charging) Medium Simple / ± 500 Short Medium Medium
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.