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An EOS-like latchup failure occurred in a high-voltage IC product during latchup test and was identified within ESD diodes themselves. A parasitic npn bipolar formed by ESD protection diodes was trigger-activated and produced large current to result in EOS failure. This was verified by electrical measurement from TLP and curve-tracer as well as physical failure analysis. Corresponding layout solutions were proposed and solved this anomalous latchup failure successfully. Therefore ESD protection diode should be laid carefully for true latchup-robust design.