NXP Semiconductors PCA8565, PCF85x3, PCF2123, PCA2125 user manual Charging the backup capacitor

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

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:

•	VCbackupstart	: The backup capacitor voltage when backup starts.
•	VCbackupend	: The backup capacitor voltage when backup ends, which equals the
	minimum oscillator operating voltage, specified in Table 2 as clock operating voltage.
•	IRTC	: The time keeping current consumption of the RTC. For lowest current
	consumption disable CLK-OUT.
UM10301_1			© NXP B.V. 2008. All rights reserved.
User manual		Rev. 01 — 23 December 2008	34 of 52