NXP Semiconductors UM10301 PCF2123 - Manual (Page 34)

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 34 of 52
If backup is only needed for a few minutes to deal with short interruptions in power, it is
possible to use a small inexpensive electrolytic capacitor.
Supercaps can not be bought from as many vendors as ordinary electrolytic capacitors.
They are available from such vendors as Panasonic, AVX and Cornell Dubilier. Important
specifications are working voltage and leakage current. If the rated working voltage is
only slightly exceeded, lifetime may be reduced. The leakage current should be as small
as possible. A standard electrolytic capacitor has a leakage current several times larger
than the timekeeping current consumption of the RTC and will limit the backup time
severely. Also leakage current of super capacitors can easily exceed the timekeeping
current consumption of an RTC and careful selection will result in longer backup time.
In most applications the lifetime of a supercap will exceed the lifetime of a NiCd or NiMH
battery. It decreases however with increasing temperature, humidity, applied voltage and
current. Although a supercap will often be the better choice as backup source compared
to rechargeable batteries in terms of available backup time, life time and cost (both for
relatively short backup times), for every specific application pros and cons of both must
be evaluated.
13.3.1 Charging the backup capacitor
Although not strictly necessary it is advised to charge the capacitor via a resistor in order
to limit the charge current. A resistor in series with a capacitor creates an RC-time
constant
Τ. In order to calculate the charging time of the capacitor the following
parameters are important:
• Capacitor value (i.e. 1 F)
• Capacitor starting voltage (i.e. 0 V)
• Series resistor (i.e. 4.7 kΩ)
The time constant T of the circuit equals R·C. The capacitor can be considered charged
after a time t = 5T. For this example t = 5 x 1 x 4700 = 23500 seconds. This is about 6.5
hours. This is the theoretical charging time of a capacitor with series resistance, but for a
supercap it may take even longer to become fully charged due to the many internal
series resistances with various values.
In this example the capacitor is charged to the supply voltage. Since the time keeping
voltage is lower than the supply voltage that is used in a typical application, it does not
take a time t = 5T for the capacitor to reach a voltage where it can start backing up the
RTC if main power would be interrupted.
13.3.2 Estimation of backup time with capacitor
In order to keep the calculations simple a constant current draw of the RTC is estimated
also when the supply voltage drops as the capacitor gets discharged. It is assumed that
the capacitor is fully charged. The following data is necessary for the calculations:
• V
Cbackupstart
: The backup capacitor voltage when backup starts.
• V
Cbackupend
: The backup capacitor voltage when backup ends, which equals the
minimum oscillator operating voltage, specified in Table 2 as clock operating voltage.
• I
RTC
: The time keeping current consumption of the RTC. For lowest current
consumption disable CLK-OUT.