Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

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. 

 

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. 

 

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.