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I have question about an ESD protection circuit, as seen below, taken from a TI document; enter image description here

My questions are about the additional zener diode on the right attached to the USB lines: 1. Is this for specific ESD protection on the USB lines? 2. If so, i've seen ESD circuits without this zener and just diodes, so what does this do and why is it needed?

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    \$\begingroup\$ What "TI document" is this from? \$\endgroup\$ – Matt Young Feb 23 '17 at 1:53
  • \$\begingroup\$ What "additional zener diode on the right attached to the USB lines"? What USB lines is a zener connected to? \$\endgroup\$ – Andy aka Feb 23 '17 at 9:27
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The Zener turns on when the inputs rise (Vzener + Vdiode) above GND.

If 6volt zener (a typical number, for highly doped junctions) + 1volt Diode (at human-body currents of 1amp), as Vesd rises to 7 volts, the Zener comes into action.

The ESD energy has to be dissipated DEEP into the substrate; the ESD design task included steering the current flows into deep resistive regions and dumping that energy.

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  • \$\begingroup\$ By 'steering the current flows into deep resistive regions' do you mean into the zener? You mentioned a 1amp human body current, is this a typical number for modelling human esd discharge? \$\endgroup\$ – ConfusedCheese Feb 23 '17 at 10:00
  • \$\begingroup\$ HBM is 2,500 volts (or more) with little or no inductance, but a crucial 1,500Kohm resistor (the human body) to limit the current. On other hand, machine model assumes little or no resistance (its metal touching metal). And the more recent charged-body-model lets a spark jump to ANY CONCEIVABLE point of the circuit/IC, not thus to the pins. And yes, the zener to be a survivor of multiple ESD hits (as required in validation) must unload heat into the silicon UNDERNEATH, thus the path for current flow ..... must be underneath, "deep", and not along surface implant structures which are shallow. \$\endgroup\$ – analogsystemsrf Feb 23 '17 at 17:50

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