0.68 µF, 25 V molded tantalum — what it fits
The TAJS684K025RNJ: This is a KYOCERA AVX TAJ series molded tantalum capacitor, 0.68 µF ±10% tolerance, rated at 25 V DC, in a 1206 (3216 Metric) surface-mount case.
ESR and temperature — the derating decisions
The 10 Ohm ESR is the figure that sets ripple-current capability and output ripple voltage. At 25 °C ambient, the part can handle a certain ripple current; above 85 °C the voltage derating curve from the TAJ series datasheet applies. For a 25 V rail, a typical design guideline is to derate to 50-60% of rated voltage at 85 °C, which means staying below 12.5-15 V applied across temperature.
