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 | 19 of 52 |
