NXP Semiconductors PCF85x3, PCF2123, PCA8565, PCA2125 user manual NiCd and NiMH secondary batteries

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NXP Semiconductors

UM10301

 

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

evaporation can be neglected. At elevated temperatures of for example 60 °C electrolyte evaporation will be much higher. Refer to the manufacturer’s datasheet. Vendors of lithium batteries include Panasonic, Sanyo and Varta.

13.2 NiCd and NiMH secondary batteries

The well known Nickel-Cadmium and Nickel-Metal Hydride batteries can also be used to provide backup power to an RTC. In many countries use of NiCd batteries will be restricted in favour of NiMH batteries due to environmental considerations (cadmium). NiMH batteries also suffer less from the memory effect than NiCd batteries. As a further plus, NiMH batteries provide a higher energy density than NiCd batteries, but on the other hand have a higher self discharge rate (about 20 % per month at room temperature) than NiCd batteries (about 10 % per month). The timekeeping current draw of an RTC is so low that often – depending on the selected battery capacity - the self discharge is the determining factor for the available backup time, which in that case would make the NiCd more suitable for backup applications. The typical operating temperature range during charging is approximately 0 °C to +40 °C. During discharge the permitted operating temperature range is a bit wider, in the order of -10 °C to +50 °C. Just like lithium cells NiCd and NiMH batteries must be separately soldered or placed in a battery holder after the board has gone through reflow soldering. The charging circuit for NiCd and NiMH batteries in this application can be very simple; just trickle charge it via a resistor or other form of current limiting. Ordinary NiMH batteries are less suitable for trickle charging than NiCd batteries which is another reason that often NiCd batteries are better in this application. However, as pointed out before, use of NiCd batteries will be restricted in many countries due to environmental considerations. Therefore it will be harder to find NiCd batteries for backup purposes. They are being replaced by newer NiMH batteries in the same form factor and which are suitable for trickle charging. An application diagram is given in Fig 13.

VSUP

D1

R1

C1

3.6 V to 4.8 V 100 nF NiCd/NiMH

VDD

RTC

VSS

001aai848

(1)Due to the low RTC current consumption, a parallel diode over R1 (directed from the battery to the RTC) will not be of any use. The voltage drop over R1 is small

Fig 13. Backup circuit using secondary cell (NiCd or NiMH)

The capacity of a battery is expressed as C. The charge or discharge current can now also be expressed in relation to the capacity of the battery. Assume a battery with a capacity of one ampere-hour (1 Ah). A discharge current of C/10 now equals 1 Ah / 10 h = 100 mA. The recommended charge current is also specified as a fraction of C.

UM10301_1

 

© NXP B.V. 2008. All rights reserved.

User manual

Rev. 01 — 23 December 2008

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Contents Abstract Info Content KeywordsDocument information Contact information NXP SemiconductorsRevision history Rev Date DescriptionIntroduction Features Register overview PCF8563 Address Register name BitEvent counter mode ComparisonFeatures Comparison of six real time clocksVoltage-low detector Power-on reset PORPower-on reset Oscillator-stop detection OscillatorVoltage-low detection Pierce Oscillator equivalent diagram Overview of internal and external oscillator capacitorsOscillator frequency determining components UM10301 + C Typical values for crystal and surrounding capacitors Parameter Value Unit SourceUsing an external oscillator Oscillation allowanceCrystal and crystal selection Effect of temperature Modes which don’t work− f nom Capacitors and capacitor selection Accuracy Influences on time accuracy Oscillator tuning Oscillator tuning 10.1 PCF2123 Offset register Century and leap year, Daylight Saving Time Century trackingYear and leap year tracking Daylight Saving Time DSTBlock Diagram PCF8563 Initialization and setting of alarm and timerInitialization of the RTC and setting the time Alarm Setting the alarmBinary BCD Register Comments AddressAlarm function Register Setting the timerSetting the timer Backup power supply Lithium Primary cellsBackup circuit using primary lithium cell Backup circuit using secondary cell NiCd or NiMH NiCd and NiMH secondary batteries13.3 Capacitors Charging the backup capacitor Diode selection Some suggestions for diode D11N4148 BAS716 BAS116 BAV170PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Hints to keep power consumption low Protection diodes0007 8473 ⋅ C b Rpmax as a function of bus capacitanceApplication diagram 1, I2C-bus interface Application diagram 2, SPI interfaceFirst period inaccuracy when using the timer Timer delaysTimer Source clock frequency Delay for n = General countdown timer behaviourFirst period delay for timer counter value n Timer source clock Minimum timer period Maximum timer periodTiming requirements for I2C read and write Block diagram I2C interface and Time counters I2C interfaceSequence of events example Read Troubleshooting Oscillator startup time Checking for oscillationNo communication via I2C-bus References Wrong time and date, wrong clock speedLegal information DefinitionsDisclaimers TrademarksContents
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