4.7 µF, 20 V, 4 Ohm ESR — decoupling and filtering ceiling
The TAJA475K020Y: This is a 4.7 µF molded tantalum capacitor rated at 20 V with a 4 Ohm ESR, in the TAJ series from KYOCERA AVX. The 4 Ohm ESR is the key derating parameter: it limits the ripple current the part can handle before self-heating raises the internal temperature above the 125°C ceiling. For a 100 kHz buck converter output, the ripple current through this cap should stay under the thermal budget — the ESR sets the I²R loss, not the capacitance alone.
A downhole tool or satellite power bus that sees -55°C startup and 125°C during re-entry can use this cap without derating the voltage below 20 V.