3.3 V rail ESD clamp with sub-pF loading
The ESD9L3.3ST5G is a single-channel Zener TVS diode from onsemi designed to protect 3.3 V signal or power rails against ESD transients. Its 3.3 V (Max) reverse standoff voltage means the diode stays off below that rail; conduction starts when the line exceeds 4.8 V breakdown. Capacitance is 0.5 pF at 1 MHz — low enough to keep high-speed signal edges clean on interfaces like USB 2.0, HDMI, or Gigabit Ethernet. The SOD-923 package (0.6 x 0.3 mm footprint) saves board real estate but requires precise solder paste deposition; rework is challenging without a microscope station.
The 9 V clamp at 1 A (8/20 µs) sets the upper voltage the downstream IC must survive. If the IC's absolute maximum rating on the pin is 5 V, this diode alone does not guarantee protection — a series resistor or secondary clamp may be needed. The 1 A peak pulse current rating is typical for IEC 61000-4-2 ESD strikes at ±8 kV contact; for higher-energy surges (e.g., 10/1000 µs), the diode is not rated.