Nxp Semiconductors PCA2125 User Manual

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

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. 

 

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.