NXP Semiconductors PCA2125, PCF85x3, PCF2123, PCA8565 user manual Capacitors and capacitor selection

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NXP Semiconductors

UM10301

 

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

handle this temperature. Generally metal can quartzes are not recommended for high temperatures because the termal cycling (expansion of package) will cause leakages in the hermetically sealed package. Micro Crystal of Switzerland manufactures a wide range of crystals which include crystals designed to operate up to 125 °C.

8. Capacitors and capacitor selection

The influence of temperature on the accuracy of the RTC application due to the temperature coefficient of the capacitances CIN and COUT is far less than due to the temperature coefficient of the crystal. Nevertheless it is good to be aware of some differences between the various types of capacitors (dielectric) around.

Ceramic capacitors tend to have low inductance because of their flat plate construction. Most other types of capacitor are wound and thus inductive. Nowadays SMD capacitors are dominant in small signal applications.

The EIA (Electronic Industries Alliance) has issued EIA-535 which defines capacitor dielectric classes. Class I and Class II dielectrics have been defined. Within these classes several types of dielectric exist. The most common ceramic types are C0G/NP0, X7R, Y5V and Z5U but others exist too.

C0G (EIA) or NP0 is the highest quality of these with the lowest capacitance / temperature dependence (Negative-Positive Zero), but has a lower permittivity, which means that its capacitance range is more restricted. NP0 refers to the shape of the capacitor’s temperature graph and for NP0 this graph is nearly flat. It also exhibits a negligible capacitance and dissipation factor change with voltage or frequency.

X7R is a reasonably stable high-permittivity dielectric which allows capacitance values up to 1μF into a reasonable package. The available range is in the order from 100 pF to

22 μF in SMT, larger values are available in leaded packages. X7R formulations fall into EIA Class II materials. X7R is the most popular of these intermediate dielectric constant materials. Its capacitance variation as a function of temperature is within ±15 % from -55 °C to +125 °C. This capacitance change is non-linear and therefore difficult to express in ppm/°C since it changes over the temperature range. Capacitance for X7R varies under the influence of electrical operating conditions such as voltage and frequency. This rules out many applications, leaving only the general purpose applications like coupling and decoupling. The leakage current is sufficiently low.

Y5V formulations are for general-purpose use in a limited temperature range. Available range is from 1 nF to 22 μF in SMT, larger in leaded packages. They have a wide capacitance change of +22 % to –82 % over the operating temperature range of –30°C to +85°C. As an example, at 31% of the rated voltage (5 V over a 16 V capacitor) the resulting capacitance will have reduced to a quarter of the rated value. The effective decoupling capacitance present may thus be much less than expected. Y5V’s high dielectric constant allows the manufacture of the highest capacitance value in a given case size. These characteristics make Y5V ideal for decoupling applications within limited temperature range. When specifying the values, the dependence on temperature and applied voltage must be taken into account.

Z5U shows in comparison to the previous types a much worse performance. Its capacitance changes by over 50 % with changes in temperature and applied voltage. Its temperature range is only +10 °C to +85 °C. Its initial tolerance can be as high as -20 %

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