NXP Semiconductors PCA2125, PCF85x3, PCF2123, PCA8565 Backup circuit using primary lithium cell

Page 31

NXP Semiconductors

UM10301

 

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

The mentioned IEC/UL standard states that circuits employing lithium batteries shall be designed to prevent forced charge and discharge if this would result in a hazard. Practically this means that the application must ensure that both charging and discharging currents will be limited to safe values under any circumstances in order for the application to pass Underwriters Laboratories safety approval, or other similar standards. If a series diode is added meeting full UL requirements is not difficult. An example schematic is given in Fig 12. Further refer to the relevant UL/IEC documents and the specification of the battery used.

D2 R1

optional

3.0V to 3.6 V lithium

VSUP

D1

C1

100 nF

VDD

RTC

VSS

001aai847

(1) D2 may not always be necessary. Refer to text.

Fig 12. Backup circuit using primary lithium cell

3.0 V or 3.6 V Lithium batteries are suitable and sizes are available that can power an RTC for over 10 years. The battery can simply be connected via a diode D2 to VDD of the RTC and ground. If for D2 a Schottky diode is chosen, voltage drop is limited. However, since the voltage of a lithium cell remains rather stable over its life time this is usually not necessary. Depending on the soldering method used the battery can often only be placed after the board has been soldered to avoid short circuiting of the battery during the soldering process or damaging the lithium cells due to the high temperatures that occur during soldering; cell temperature must remain typically below 85 °C. Therefore a holder must be provided in which the battery is placed after soldering, or the battery must be soldered separately on the board after the other components have been placed. This increases cost.

Self discharge at room temperature and below is typically less than 1% per year. At higher temperatures, say above about 60 °C, self discharge increases quickly. Obviously, this self discharge also occurs when the RTC is not battery powered, the lithium cell is always there. Therefore the storage and operating temperature of the application is to be considered as well. During battery discharge the voltage remains stable such that at the end of life the voltage is almost the same as with a fresh battery and then suddenly dropping fast.

Many countries govern disposal of electronics products including the batteries at end of life. In some cases the manufacturer is responsible for complying with such regulations which may need some attention during design of the product.

In order to calculate the possible backup time, based upon the current consumption of the RTC, divide the cell capacity in ampere-hours by the timekeeping current draw of the RTC. For example, a BR1220 battery with a capacity of 35 mAh would have a theoretical life time while supplying 250 nA to a PCF8563 of 35 mAh/250 nA = 140 000 hours. This equals about 16 years. However, this is only true at room temperature where electrolyte

UM10301_1

 

© NXP B.V. 2008. All rights reserved.

User manual

Rev. 01 — 23 December 2008

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Contents Document information Info Content KeywordsAbstract Rev Date Description Contact informationNXP Semiconductors Revision historyIntroduction Features Address Register name Bit Register overview PCF8563Comparison Event counter modeComparison of six real time clocks FeaturesPower-on reset Power-on reset PORVoltage-low detector Voltage-low detection OscillatorOscillator-stop detection Overview of internal and external oscillator capacitors Pierce Oscillator equivalent diagramOscillator frequency determining components UM10301 + C Parameter Value Unit Source Typical values for crystal and surrounding capacitorsOscillation allowance Using an external oscillatorCrystal and crystal selection Modes which don’t work Effect of temperature− f nom Capacitors and capacitor selection Accuracy Influences on time accuracy Oscillator tuning Oscillator tuning 10.1 PCF2123 Offset register Daylight Saving Time DST Century and leap year, Daylight Saving TimeCentury tracking Year and leap year trackingInitialization of the RTC and setting the time Initialization and setting of alarm and timerBlock Diagram PCF8563 Register Comments Address AlarmSetting the alarm Binary BCDAlarm function Setting the timer Setting the timerRegister Lithium Primary cells Backup power supplyBackup circuit using primary lithium cell NiCd and NiMH secondary batteries Backup circuit using secondary cell NiCd or NiMH13.3 Capacitors Charging the backup capacitor BAS716 BAS116 BAV170 Diode selectionSome suggestions for diode D1 1N4148PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Protection diodes Hints to keep power consumption low0007 Rpmax as a function of bus capacitance 8473 ⋅ C bApplication diagram 2, SPI interface Application diagram 1, I2C-bus interfaceGeneral countdown timer behaviour First period inaccuracy when using the timerTimer delays Timer Source clock frequency Delay for n =Timer source clock Minimum timer period Maximum timer period First period delay for timer counter value nTiming requirements for I2C read and write I2C interface Block diagram I2C interface and Time countersSequence of events example Read Oscillator startup time Checking for oscillation TroubleshootingNo communication via I2C-bus Wrong time and date, wrong clock speed ReferencesTrademarks Legal informationDefinitions DisclaimersContents
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