What this ESD diode protects — and what it doesn't
The ESD5Z5.0T5G is a single-channel Zener-type ESD protection diode from onsemi, designed to clamp transients on general-purpose digital I/O, power, and signal lines operating at or below 5V. Its 5V max reverse standoff voltage means it sits on a 3.3V or 5V rail without conducting leakage, then breaks down at 6.2V min to shunt the surge to ground. Peak pulse power is rated at 174W for an 8/20µs waveform, which covers IEC 61000-4-2 ESD strikes up to ±15kV air discharge typical for this class. The 80pF capacitance at 1MHz adds about 20pF to the line — fine for I2C, UART, or USB 1.1, but too much for high-speed differential pairs like USB 2.0 or HDMI.
Clamping voltage and peak current — sizing the protection margin
Maximum clamping voltage is 18.6V at a peak pulse current of 9.4A (8/20µs). That clamping voltage is the voltage the protected IC sees during the event — a 3.3V CMOS input rated for 5V absolute max has about 13V of headroom, so this diode provides adequate protection for most 3.3V logic families. If the downstream IC has a lower absolute maximum rating (e.g., 3.6V), a lower-clamping part is needed.
onsemi lists the ESD5Z5.0T5G as Active.
Package and footprint — SOD-523 layout notes
SOD-523 (SC-79) is a two-lead package with a 0.65mm pitch and 1.2mm body width. No exposed thermal pad; the junction-to-ambient thermal resistance is set by the copper pad area on the PCB.