NXP Semiconductors UM10301 PCF2123 - Manual (Page 33)

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 33 of 52
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 V
SUP
= 5.5 V, then R
1
= (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 V
DD
and V
SS
is possible, but it is recommended to use a series resistor
R
1
. 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.