HP-71B 4Kb (CMT32k FrontPort) Memory Module "Battery" - FRAM

[Garth Wilson]

Note that the piezoelectric effect on MLCCs is huge, and the capacitance drops off rapidly above approximately 20% of the WVDC. The dielectric type has an effect too, Y5V being considerably worse than X7R. A Y5V will lose over 85% of its capacitance at its WVDC, whereas the X7R only loses 70%. There's an article on this in EDN magazine, Apr 12, 2007, starting on page 77. (Would it be a no-no to post it here?) So for a power backup capacitor for the module, you'll want to get the largest capacitor that will fit mechanically, a 25V (5 times 5V) X7R capacitor with the highest capacitance that will fit.