NXP Semiconductors PCF2123, PCF85x3, PCA8565, PCA2125 user manual Capacitors

NXP Semiconductors			UM10301
		User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
	The recommended trickle charge current for a NiCd or NiMH is expressed as a fraction of
	C and is typically in the range C/50…C/20. Refer to the datasheet of the selected battery.
	A too small trickle current will not properly keep the battery fully charged, a too high
	current leads to overcharging and this will limit the life time of the battery. A diode D1 is
	necessary to prevent the backup battery from supplying circuitry other than just the RTC.
	Battery life time is limited by the number of charge / discharge cycles. Typically after 500
	cycles the capacity has dropped to 60% of the original capacity which is defined as end
	of life for the battery.
	As an example the selected battery (Varta 3/V150H) is a 3.6 V NiMH type with a capacity
	of 150 mAh. The recommended trickle charge current is 4.2 mA. This equals C/36. If the
	supply voltage VSUP = 5.5 V, then R1 = (5.5 – 0.5 – 3.6) V / 4.2 mA = 330 Ω. At such small
	currents, the forward voltage of a diode is less than 0.7 V and was assumed to be 0.5 V.
	Vendors of NiCd and NiMH batteries include Panasonic, Sanyo and Varta.
13.3	Capacitors
	In order to provide backup power to applications like RTC and volatile memory a special
	type of capacitor (supercap) was developed which combines a high capacitance, low-
	leakage and relatively small dimensions. Common values are a few mF up to 1 Farad
	and sometimes even more. These supercaps do not have a dielectric like ordinary
	capacitors but use a physical mechanism that generates a double electric field which
	acts like a dielectric. They are also referred to as EDL capacitors, Electric Double Layer
	capacitors. Charging-discharging occurs in the ion absorption layer which is formed on
	the surfaces of the positive and negative electrodes. Manufacturers of this type of
	capacitor use two types of electrolyte. One is water-soluble and the other is non-water-
	soluble. The latter can withstand higher voltage per cell. This type of capacitor thus uses
	special techniques in order to achieve such high capacitance values in a compact
	package. Just like an electrolytic capacitor it is polarized.
	The equivalent circuit consists of many RC series circuits, connected in parallel. The
	capacitor is comprised of many small capacitances having resistances of various values
	in series. Therefore the current through the capacitor can be regarded as the sum of the
	currents flowing through each of the small capacitors. Due to the internal resistances, it
	will take many hours before the capacitor is fully charged, also when it is connected
	without external series resistor directly to a voltage source.
	Capacitors are used for applications where backup power needs to be provided for
	relatively short times. Due to the low current consumption of an RTC it is possible to get
	several weeks of backup operation using a supercap. Using the PCF2123 or PCF8563
	with a value of 1 F or 1.5 F will even result in several months of backup time. Advantages
	of a capacitor over batteries are amongst others the ability to be soldered together with
	the other components in wave or reflow soldering. There are also no regulations
	regarding disposal because they don’t contain any heavy metals. Just like the NiCd or
	NiMH batteries, the capacitor needs to be charged during normal operation and will
	provide backup power when the application is off since the RTC still needs to keep track
	of time. The application is very simple, refer to Fig 13. Here the supercap takes the place
	of the battery. Connecting the supercap directly in parallel with the normal decoupling
	capacitor between VDD and VSS is possible, but it is recommended to use a series resistor
	R1. The capacitance of the capacitor will change over its lifetime, especially at higher
	temperatures. An increase of temperature by ten degrees results in a 50% decrease of
	lifetime. Therefore be sure to specify extra backup time initially to allow for this decrease.
UM10301_1			© NXP B.V. 2008. All rights reserved.
User manual		Rev. 01 — 23 December 2008	33 of 52