NXP Semiconductors UM10301 PCF2123 - Manual (Page 31)

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 31 of 52
The mentioned IEC/UL standard states that circuits employing lithium batteries shall be
designed to prevent forced charge and discharge if this would result in a hazard.
Practically this means that the application must ensure that both charging and
discharging currents will be limited to safe values under any circumstances in order for
the application to pass Underwriters Laboratories safety approval, or other similar
standards. If a series diode is added meeting full UL requirements is not difficult. An
example schematic is given in Fig 12. Further refer to the relevant UL/IEC documents
and the specification of the battery used.
001aai847
R1
D1
D2
3.0 V to 3.6 V
lithium
optional
C1
100 nF
V
SUP
V DD
V SS
RTC
(1) D2 may not always be necessary. Refer to text.
Fig 12. Backup circuit using primary lithium cell
3.0 V or 3.6 V Lithium batteries are suitable and sizes are available that can power an
RTC for over 10 years. The battery can simply be connected via a diode D
2
to V
DD
of the
RTC and ground. If for D
2
a Schottky diode is chosen, voltage drop is limited. However,
since the voltage of a lithium cell remains rather stable over its life time this is usually not
necessary. Depending on the soldering method used the battery can often only be
placed after the board has been soldered to avoid short circuiting of the battery during
the soldering process or damaging the lithium cells due to the high temperatures that
occur during soldering; cell temperature must remain typically below 85 °C. Therefore a
holder must be provided in which the battery is placed after soldering, or the battery must
be soldered separately on the board after the other components have been placed. This
increases cost.
Self discharge at room temperature and below is typically less than 1% per year. At
higher temperatures, say above about 60 °C, self discharge increases quickly. Obviously,
this self discharge also occurs when the RTC is not battery powered, the lithium cell is
always there. Therefore the storage and operating temperature of the application is to be
considered as well. During battery discharge the voltage remains stable such that at the
end of life the voltage is almost the same as with a fresh battery and then suddenly
dropping fast.
Many countries govern disposal of electronics products including the batteries at end of
life. In some cases the manufacturer is responsible for complying with such regulations
which may need some attention during design of the product.
In order to calculate the possible backup time, based upon the current consumption of
the RTC, divide the cell capacity in ampere-hours by the timekeeping current draw of the
RTC. For example, a BR1220 battery with a capacity of 35 mAh would have a theoretical
life time while supplying 250 nA to a PCF8563 of 35 mAh/250 nA = 140 000 hours. This
equals about 16 years. However, this is only true at room temperature where electrolyte