Passive Components


Replacing tantalum with high capacitance MLCC

13 February 2002 Passive Components

For several years there have been periodic variations in availability and price of tantalum capacitors. In the past this behaviour was due to political and economic strategies. Today, it is the poor supply of the tantalum raw material that is driving the Ta-capacitor crisis. At present we are seeing price growths up to 500% and the forecast is for further hikes. To help solve these difficulties Murata offers many ceramic capacitors that can directly replace electrolytic and tantalum types.

Figure 1 shows some basic circuits that need high capacitance values:

* Smoothing: The functions of C1 and C2 are to smooth the ripple and voltage fluctuations at the input and output of the LDO (low drop out regulator). C2's ESR and ESL are most important because they are responsible for the purity of the output voltage. In the past, high value Ta capacitors were used; now it is possible to use ceramic capacitors at a half to a tenth of the Ta values used previously.

* By-passing: C3 creates a 'virtual' ground for the transistor which 'believes' it is working at ideal conditions. The static and dynamic parameters are satisfied and the active device is properly used. The ESL of the cap is also most important in this case, and a low value avoids self-oscillation problems.

* Coupling: In order to link two stages (for example pre-amp to power amplifier) C4 is basic. This capacitor transfers only the signal and does not modify the DC parameters. For the example mentioned, it is most important that the capacitor is not polarised in order to avoid signal distortion.

What better solution than a ceramic capacitor? For more information on the technical characteristics of MLCCs and products available from Murata, contact your local distributor.

For further information contact Avnet Kopp, (011) 444 2333, sales@avnet.co.za or www.avnet.co.za



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